Frenchy: Those one-use credit cards can come with a caviot. Many vendors will put through a "test charge" before really billing the card. So if you plan to donate $10, and create the card for only $10, then it may get denied because a test charge of a dollar or so was first attempted, and then the one-use card did not have enough remaining money to cover the real $10. You are not actually billed for these test charges, but they count against the maximum amount you allowed for the card. Some merchants even test with the full amount of the purchase, so you have to double the amount you really want to spend when you create the one-use card.

I usually create a card for $10 over what I really need. That goes through most of the time. When it doesn't, I increase the amount on the card to double the expected purchase amount.