Hey, thank you for the feedback 
Let me clear some things up!
Emails are not self deleting in some Mission Impossible ’ This email will self destruct after 10 seconds’ kind of way. The original customer most likely has some settings in his email client which auto deletes emails.
In an ideal world each person would have one email address which they keep forever and guard the details of as if it was their personal diary, because in many ways, it is. However this is not an ideal world and both we at Greenheart Games and Fastspring understand this. Therefore we, I say we but it’s mostly Fastspring, as that’s what they do, have simplified the procedure as much as possible.
To regain a purchased product’s details simply contact orders@fastspring.com. Your identity is your proof of purchase. After confirming your identity they will resend any pertinent information within 72 hours, often within 24. You can also contact me via support@greenheartgames.com.
There is an entire section dedicated to this in the pinned FAQ.
Creating folders inside your email client named ‘Invoices’, ‘Amazon’, ‘Games’ etc and then moving important emails to these folders is a great way to stay organised.