PayPal won't accept the card I have, and I wanted to say "thank you" for all the work on Ubuntu with, well, some cash. How else can I pay?
If PayPal does not accept your card - maybe your country is not supported or any other reason - I do not believe you can donate any other way. At least not that I know of.
There is an alternative though.
You can visit the Canonical shop and buy something at http://shop.ubuntu.com
There is no need to use PayPal at the shop for your purchases.
Also, the link to the shop is available at the top of this website, and any other Ubuntu affiliated website.