You can buy the PSN card things from your local brick and mortar store. I know that's more of a hassle than just putting your credit card in directly, but it will do the job... and truth be told it is a lot more secure. Don't forget how hackers broke into PSN a year ago and made off with millions of credit cars numbers. Do you really trust your information to be on PSN?
As someone who makes fun of how bad the PS3 is on a daily basis, of course I am familiar with its shortcomings.
I really don't want to buy a PSN funny money card because
1) I'd probably have to get one from the US, where I no longer live
2) The amounts are like 10 dollars, 20 dollars, etc. I only want to spend 15 and don't want to give Sony any more money, ever
3) tahts gay/