What to do, when after purchase you don’t get a receipt letter to your email? Because I can’t get my Order ID number 
Thanks for that, but I’ve already read all of it and nothing helps. Is there a way to ask to resend order confirmation mail or something?
Did you buy from Avast directly or from a reseller…?
Directly. I can show my transaction details if needed.
Contact support: https://support.avast.com/contact