If YOU are in the US, you should try someone like cdgcommerce who posts here regularly and seems to provide amazing service. If you are NOT in the US, I'd probably start by asking your bank to see if they provide a merchant account - if you're doing that kind of business then their small business advisor should be quite willing to chat with you for a while.
Best known third party is probably paypal (but understand limitations before using) or 2checkout.com. Search for 2checkout in the forum and you'll also get a list of competitors. But 2co is 5% plus $49 setup plus additional fees for international withdrawals, so check into fees before you sign up!
Based on what you have mentioned above, a direct merchant account with a U.S. merchant processor *seems* like it would be the most cost effective option.
Are there any issues that have been raised by processors that you have spoken with which you feel might preclude you from being approved? (i.e. credit, type of business/products, etc.)
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I would highly recommend Corey Bryant and the team at Merchant Accounts 4 Less (http://www.merchantaccounts4less.com). We used them to open our merchant account and we have been extremely happy. They also offer WHT members a discount!