I can’t seem to use my paypal account to buy games where the total price is under $5. If this isn’t a bug, then this is a suggestion to be able to use your account to buy things under $5 without pre-adding funds to your wallet. I have enough money to get a $2.36 game, but I don’t have the $10 I need to add to my wallet first to get it.
That’s not a bug. For cheaper items, it does not make a lot of sense to charge credit card or PayPal (which carry a high fee).
I also buy from Amazon, and the cheaper items are becoming “add-ons” to reduce charge & shipping cost. If you use Steam (or PSN, Xbox Live), they’re all want you to deposit funds.