It's Paypal... Which is why I don't accept it as payment at my business. I don't recall the exact fees but, it's something like 5% plus, a per-transaction fee of about 45 or 50 cents. I'm doing this from memory and my numbers might not be exact -so please be tolerant. When I looked at their terms & conditions (for about 2 minutes) I chuckled and threw the paperwork away.
The reason many places accept Paypal is because they don't do extensive credit checks on the vendors that decide to use their services. It's popular among customers because no credit card numbers need to be exposed.
FWIW, the site owner/operator here is to be trusted and nothing fishy is going on. I suspect this site does not take other credit cards because, they have fees too -which probably don't fit the business model at which this site gets it's income.
Ray