If you plan on accepting at least $3000 a month, PayPal becomes cheaper than Stripe. If you're going to be charging tens of thousands a month or more, then a merchant account with low interchange-plus pricing (that's a fixed markup over costs -- the interchange fees charged by Visa/MasterCard) is likely your best bet.
You can use Dwolla or Balanced, or work directly with a bank, to accept donations by ACH (bank transfer) as well. They cost 10-25 cents per transaction, which is much cheaper than credit card payments.
You can use Dwolla or Balanced, or work directly with a bank, to accept donations by ACH (bank transfer) as well. They cost 10-25 cents per transaction, which is much cheaper than credit card payments.