What is the best way to collect customer emails?

At checkout, because they are already giving you one. Everything else — pop-ups, competitions, discount bribes — collects addresses of people who wanted the bribe.

The reasoning

The second-best is a genuinely useful reason: a restock alert, a delivery notification, early access to something scarce. A list built from a 10% discount pop-up opens at a fraction of the rate of one built from restock alerts.

What most people get wrong

The slowest part of most mobile stores is images nobody resized. It is the most common performance problem and the easiest to fix.

Discounting to grow trains customers to wait for discounts. It works once and costs afterwards.

Traffic is the expensive problem and conversion is the cheap one. Doubling a 1% conversion rate is usually easier than doubling visitors, and costs nothing per month.

How an app changes it

Push notifications are the one growth channel you do not rent. Every other route to a customer is paid for again each time.