Authorize.net is easy, affordable and very professional.
If you are unsure of your sales volume and are just starting out, I would suggest going with PAYPAL 'Website Payments Pro'. You need a business account, I believe, to set this up - but it is very easy to set up a biz acct with paypal. Your customers do not see paypal as the one accepting the credit card. This is the cheapest method, it is $20 per month plus the percentage per transaction. There is no contract obligation either.
Once you get your merchant account setup, you will breathe a sigh of relief :)
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