I send an email with that subject to new customers 24 hours after they place an order. Well, actually, I don’t sit at my computer with a stopwatch… my autoresponder sends out the emails for me. 
It’s a simple little technique that’s really appreciated by customers. I get a lot of, “yes, no problem” replies and some “I didn’t get the download link” emails. The autoresponder company I use, and recommend, are on a different server to my websites, so if the original “thanks for your purchase” email is not received by the customer for whatever reason, they’ll still get the “did you receive your order OK” email 24 hours later… and let me know they didn’t get it… then I can personally send them download details, passwords or whatever they need.
What I like is getting replies like this one, which arrived this morning…
“Thank you for the email. Yes, I recieved everything and have started reading the book. Amazing, I am looking forward to starting some sites after I have finished reading it. The step by step instructions are just what I wanted. Many thanks.”
What was he talking about? Auto Income Secrets. 