Ok, here is some out of the box thinking. Sorry ahead of time if this is way too much effort, or if someone knows a better way.
Make a shopping cart with donation products in multiples of 5 or 10, whatever.
I use magento commerce myself, they have an option to use paypal, authorize.net, or to save the credit card numbers.
I am an affiliate, I havent really used paxum yet as I am so far a fail affiliate lol. That being said, is there a way you can use them or someone similar to deduct money from an account?
If not get a squareup account and the squareup app for your smartphone and just manually type the donations in as you get them.
Sorry, probably not the best answer but it's all I could come up with.
Best of luck!
Edit: Magento also has a donation add on you can buy that may allow you to collect whatever amount you like.
I haven't used it so I don't know how it works though.
http://www.magentocommerce.com/magen...mate-4489.html