shedonist, PayPal couldn't be simpler. I use it to receive online payments, and all the person has to do is send you the funds. You're instantly notified when PayPal receives the payment. You then log in to your account there and withdraw, either by having the check mailed, which takes one to two weeks, or by having it deposited directly into a bank account that you specify. You receive an immediate confirmation of the withdrawal by email. It takes 3 to 5 days for the funds to reach your bank account, and the small percentage is automatically deducted. You do have to call your bank to see when the payment is received, and the funds are automatically available because it's by electronic transfer.
The other party has no idea how you're handling the payment on your end, and you've got no idea how it was handled on their end. It's all handled by PayPal.
There's a history kept on their site that you can download, and you can print the details of each transaction out for your records.
You can set up a custom page on your site any way you want, and just include the small button graphic provided for people to click on to link and make the payment. It's just that simple.
Amazon charges about 5 times as much, and I see no particular advantage, except to Amazon by giving them a little extra promotion.
Added: Just re-read your post. Since it's the Advertising forum, here's a suggestion. If there are any books relevant to the organization you run the site for, you can do an Amazon affiliate program to get a little revenue. That's a whole other issue than getting contributions, but could be a possibiity for you to get a little extra.