Oh, you want to be able to accept credit card payments and not have people sign up for anything? Yeah, good luck with that. If you have that many people you're contacting and any significant number pay via PayPal via a credit card, your account is going to be pushed into the next tier, I'm pretty sure.

Your best bet is probably TextPayMe. Call it Amazon WebPay; that includes their web-based payment system, and adds an air of authenticity.

If I were you, I think I would provide people multiple ways to pay. Tell them something like, "You can pay via Amazon WebPay, credit card, or by sending a check. If you want to pay by using a credit card, I'm afraid we'll have to charge you a $x.xx fee beyond the cost of the reunion in order to cover the processing fees, but you can use a credit card to fund the Amazon WebPay payment for free."