Get them to come back with delight.
Stick a gift or discount coupon on coins. Make it a thin layer of transparent material, so it’s not easily detected when they get their change at the store. Print on it the coupon and sneak such a coin into the others you give them as change. Then watch them come back with delight.
Research have shown that what people remember most from an experience, are the highlight and the end.
After you convinced the customer to buy your product or service and had him go through the quite laborious process of doing so online, do one last surprising, kind, and delight eliciting thing. This can be a pop up with a sincere thank you and a gift – anything that causes them to feel good, even if unrelated to their purchase.