I use Avangate - and I love it. They act as Merchant of Record, take care of all sales tax headache, and with just one integration, I can accept payments in every country I have customers in, including the "difficult ones" like Brazil, India or China. Good service, too. Granted their API documentation and general user experience is a but sucky but if you start working with them, you will see why they are successful. If they were a public company I would buy their stock!