Does anyone knows what kind of device / company you could use to be the third personn of trust to receive a payment for a supplier against a commission ?

For exemple, hostelworld.com. The payment goes straight to the supplier via their website but in the same time they receive a commission for their services.

Is it easy to add to a website? Is it expensive? Does it comes to a lot of obligation and responsability ?

PS : sorry, english is not my first language...