Something else to consider is physical vs digital products. If I buy a physical product for 9.99 and I give 10, I get 0.01 back. The cashier actually gives you something back that you can put back in your pocket which makes you feel better about the purchase. This doesn't happen online though. If you charge $9, I can't pay you $10 and get $1 back. So a whole element of this psychology is missing in digital products