The first sixty seconds after someone pays
Everyone spends their time on the page before the purchase. Almost nobody rehearses the minute after it, which is a shame, because that minute is where a buyer decides whether they were right to trust you.
Three things happen in it. The buyer sees a confirmation, an email arrives, and they open the file. Any one of those going wrong turns a good product into a support thread.
So rehearse it. Buy your own product, on a phone, on a connection that is not your office wifi. Watch what the confirmation actually says. Time the email. Open the download on a device that is not the one you built it on.
The most common failure is not a broken link. It is silence: a confirmation that says "thank you" and nothing about what happens next, followed by an email that takes four minutes while the buyer wonders whether they mistyped their address.
Say what will arrive, say roughly when, and say what to do if it does not. Three sentences. It is the cheapest credibility you will ever buy.