Email and WhatsApp marketing: how to sell more to people who already bought

Many businesses spend almost all their effort getting new customers and almost none on the ones they already have. It's an expensive mistake: your current customers already trust you, so selling to them again is usually easier and cheaper. Email and WhatsApp are the two most direct tools to do it, and unlike social media, no algorithm decides whether your message gets through.

In Puerto Rico this matters even more. Almost everyone here lives on WhatsApp, and in a small market, referrals and repeat purchases make an enormous difference. This is the system I set up with small and medium businesses so retention stops depending on the owner's memory.

Build your list (with permission)

Your contact list is an asset that doesn't depend on algorithms or on whatever a platform decides next month. A few ways to grow it:

• Offer something useful in exchange for an email: a guide, a welcome discount, early access to launches or an event.
• Ask for contact details at checkout, with a clear consent checkbox for promotions (separate from the order confirmation).
• Capture at the point of sale: a QR code on the counter, the receipt or the packaging that leads to a short form.
• Invite people to message you on WhatsApp from your website, your bio and your ads, and ask in that first conversation whether they want to receive updates.

Always with permission and an easy way to unsubscribe. Besides being the law (Puerto Rico follows the same federal rules as the rest of the U.S. for email and text messages) and Meta's policy for WhatsApp, it protects your reputation.

What not to do

• Buy lists or "inherit" contacts from another company.
• Add everyone who once contacted customer service to your promotional list.
• Create a WhatsApp group with hundreds of customers without asking them. You expose their numbers to each other, and it's the fastest way to get reported.

Set up the technical foundation

Before you send your first campaign, make sure your messages will arrive:

• Email from your own domain ([email protected]), not a free account.
• SPF, DKIM and DMARC configured. Gmail and Yahoo require authentication from bulk senders, and without it your emails end up in spam or rejected.
• One‑click unsubscribe link, visible in every email.
• The WhatsApp Business app if you're a small business, or the WhatsApp Business Platform (API) through a provider if you need to automate, connect to your store or have several people answering at once.

One important WhatsApp detail: messages you start outside an active conversation are sent with Meta‑approved templates and cost money per message, with rates that vary by type (marketing, utility or authentication). So every send has to be worth it.

The automated messages that perform best

Set them up once and they work for you every day:

1. Welcome: introduce the brand, tell your story and deliver what you promised. It's usually the email that gets the most attention of any you'll send.
2. Abandoned cart: a friendly reminder a few hours later and another the next day. Answer common questions (shipping, returns, payment methods) before reaching for a discount.
3. Post‑purchase: say thanks, explain how to use or care for the product and, a few days later, ask for a review.
4. Replenishment: if your product runs out or renews, remind them at the right time. Base the timing on your own data about how long people take to come back.
5. Win‑back: give people who haven't bought in a while a reason to return: something new, a personal recommendation or an incentive.

Examples by type of business

Email vs. WhatsApp: when to use each

• Email: longer content, news, guides, catalogs and automations. Ideal for educating and maintaining the relationship at a very low cost per send.
• WhatsApp: short, personal messages, customer service, confirmations, reminders and timely offers. It gets a lot of attention, so use it carefully and don't overdo it.

My rule of thumb: email is your magazine; WhatsApp is your phone call. You call when you have something important to say, not every day.

Ideally they work together. For example, an abandoned cart can first get an email and, if there's no response, a short WhatsApp message offering help with any questions.

Rules for not being pushy

• Predictable frequency: if you promised a weekly email, send one a week. On WhatsApp, as a general rule, a few promotional messages a month are more than enough.
• Segment: don't send the same thing to someone who bought yesterday and someone who has never bought. Three basic segments already change the game: recent customers, inactive customers and subscribers who have never bought.
• Every message should add something: information, usefulness or a relevant offer.
• Personalize with their name and what they bought or are interested in.
• Respect the clock: no messages at eleven at night or on holidays without a good reason.
• Reply: if someone answers your WhatsApp, someone on your team has to be there to continue the conversation.

Common mistakes I see

• Living on discounts. If every email is "20% off," you train customers never to buy at full price.
• Sending only when sales are slow. Customers notice when you only remember them when you need money.
• Not cleaning the list. Contacts who haven't opened anything in many months hurt your deliverability. Try to re‑engage them and, if they don't respond, remove them.
• Heavy designs. An email that is one big image looks bad on a phone, sometimes doesn't load and can't be read when images are blocked.
• Automate and forget. Automations age too: review prices, links and offers at least every quarter.

What to measure

• Email clicks and conversions. Open rate is a useful reference, but today it's inflated by privacy features such as Apple Mail's, so don't make decisions on it alone.
• Sales attributed to each automation. It tells you which ones to improve first.
• Repeat purchase rate per month or quarter, and the average time between purchases.
• Unsubscribes and spam complaints: if they rise, you're sending too much or something irrelevant.
• On WhatsApp: replies, blocks and sales generated per campaign. A rise in blocks is a serious warning sign.

A 30‑day plan to get started

1. Week 1: consolidate your permission‑based contacts in a single tool and set up domain authentication.
2. Week 2: write and activate the welcome and abandoned cart flows (or the appointment reminder, if you sell services).
3. Week 3: activate the post‑purchase flow with a review request.
4. Week 4: send your first segmented campaign and set your frequency.

FAQ

Do I need the WhatsApp API, or is the WhatsApp Business app enough?

If you're a small business handling a few conversations a day, the app is usually enough at first. When you need to automate messages, connect to your store, measure campaigns or have several people answer the same number, it's worth moving to the API through a provider.

How often should I send emails?

The right frequency is the one you can sustain with content that adds value. For most small businesses, somewhere between one email a week and one every two weeks is a good starting point. Watch unsubscribes and adjust.

Can I add customers who already bought from me to my list?

You can send them messages related to their purchase, but for promotions the right thing is to ask for permission. A simple way is to invite them to subscribe with a clear benefit.

Which email tool do you recommend?

It depends on where you sell. If you have an online store, start with a tool that integrates natively with your platform so you can use purchase data. The brand matters less than being able to segment and automate.

Bottom line

A customer who buys again is the most profitable marketing there is. Build your list with permission, make sure your messages arrive, automate the key messages and use email and WhatsApp to nurture the relationship, not just to sell.

Share: LinkedIn · Facebook · X · WhatsApp

← Back to blog