Advice for E-Commerce w/all proceeds going to charity

I've got a prospective client who's an athlete, and they'd like to have an online store on the site in order to offer autographed memorabilia, etc. However, 100% of the profits will be going toward Breast Cancer Research, which I think is great.

The only thing is, this will be my first time setting up an ecommerce solution, so I'm currently looking at programs like X-Cart (php based), which can hopefully be integrated into (or around) the cms of the site (again, I'm outsourcing this to a coder... my skills are limited).

I was wondering though, do most people use third party payment systems with these stores? I was wondering how that would factor into everything with the charitable donations...

Also, any recommendations on the best e-commerce platform would be great. There would probably only be a need to sell 20-50 items, with a way to sell to international customers.

thanks!

could approach people like Worldpay and Protx to see if they'd be willing to offer a concession on normal rates.
Paypal is good for international in a cheap and cheerful way...

What country it the client in who will own the merchant account? If he is in the US, chances are a merchant account with an electronic payment gateway would be the best option.