I realise this has been discussed, but I want to bump the discussion. This is an absolutely essential feature in the system for a number of reasons:
1. User may have entered an incorrect email address, and there is only one chance to do so.
2. User can claim they never got a receipt, and later look for refund etc.
As this must be a fairly straight forward feature to implement, I think it needs to be on the product roadmap in the short term.
Many thinks for the great system.