WePay (https://www.wepay.com/) is a YC alumni company that seems to be doing well as a sort-of alternative to Paypal. Paypal can be fickle; if you never have trouble with them, they're convenient and easy and everyone knows who they are so you don't have to worry about convincing people that it's OK to use them. On the other hand, there are a lot of Paypal horror stories.