I have the same problem with my store
I have the same issue with orders not processing to the data base. The registration process works and orders do flow through the system which I also only know about because PayPal does provide a receipt when you make a sale.
Since orders don't post to the database nothing down stream works either. The customer does not get a confirmation email, etc.
I haven't touched any code so this must be a problem inherent to the program. I can't believe every customer hasn't this problem. Version 4.5 downloaded 8-20-11.
I want to set up another store while the program is on sale but can't justify buying another copy when this doesn't work.