The goal of your post-purchase sequence is to nurture existing customers (treat them like VIPs), keep them accountable, and eventually down-sell or upsell.
If you’re usig Gumroad you can automate a post-purchasing sequence once people buy your product without any external email platforms...
If you’re using other email marketing platforms, some of them, like ConvertKit, allow you to integrate with most shopping cart software and send a post-purchase sequence when they buy your product.
As far as the others go, you can integrate your shopping cart with your email marketing platform with Zapier.
Post-purchase framework
Here's a simple framework you can swipe...
Email 1 (immediately): Thank people for purchasing, congratulate them on their good decision, and tell them what to expect from their exprience.
Email 2 (72h delay): Ask your reader if he needs help and ask for feedback (and for them to rate your product in your sales page.)
Email 3 (72h delay): Make your reader accountable and upsell them your service (done-for-you or consulting.)
Email 4 (168h delay): Down-sell another paid product that complements the one they've bought.