I decided that I was going to offer this service to my customers on my website. (its still in development, done in about 2 weeks) But google checkout seems like the best deal. No monthly fees, flat fee of 2%, can email invoices or pay online via shopping cart. I think this is a great place to start. If you have heavy volume, I would create a custom "online cart" with a developer and find a credit card company to process the payments. The benefit to this would be overall lower costs in terms of processing charges. It is a great option for for customers to have.