There's two ways I can go about that. I can take paypal or bitpass. Paypal is the bigger system, but it also has the more soul-crushing license and more horror stories of their "seller protection" services, and how they really don't work on the off chance that they're needed. There's also no intrinsic support for donations. Everything is a direct transaction. On the other hand, by being a corporate giant, more people have paypal than any other easy means of online payment. There's also
Bitpass is a pre-paid account that you can use at places like Mperia. Users can transfer money to their account from a paypal account, can donate however much they like (Paypal's fees make donations of less than a dollar pointless). The system is set up for donations inherently as opposed to transactions. On the other hand, it's another system to set up an account in, another place to remember a username and password, and more information to keep track of. There's a smaller userbase, and I'd not be surprised if nobody reading this had a bitpass account.
If other, please specify