We see a lot of retailers who wait until the purchase to try to gain permission to market to their customers. This is problematic because we've all had the experience of receiving a barage of emails following a purchase, and often unsubscribe then and there.
The smart retailers use a simple light box to get permission the very first time someone comes to the site.
Think about it... you're paying Google or someone a hefty fee for that click to get someone to your site. They are on your site because something you did intrigued them in your brand, your products, your service... whatever it is, they are interested at that moment.
What this does is set you up for success because you now have permission to market, and once they've made a purchase, you can associate that purchase data with an opted-in subscriber for some really powerful marketing (post purchase welcome series, post purchase thank you series, replenishment campaigns, win-back campaigns, etc).
Here are two examples I came across recently, and thought I'd share them.