Most processors have resellers that you go thru actually. Some of the larger processors are Cardservice, Nova, Echo, etc.
There are two components you need: a merchant account & an electronic payment gateway (LinkPoint, Authorizenet.com, Verisign e.g.).
With a merchant account provider (MAP), you can usually be looking at a discount rate of about 2.39%, a transaction rate of about $.30. Usually there is a monthy fee (customer service, statement, etc) of about $10-$30. Most have a monthly minimum - usually around $25. But as long as you are processing (usually) over $1,050, you will not be subject to that monthly minimum. This is usually dependent on the discount rate.
As far as the gateways, the fees are usually about $15-$30 a month. Some of the gateways will also change a transaction fee as well - usually $.10-$.15.
The LinkPoint payment gateway is probably one of the strongest out there. It is owned by the First Data Corporation. First Data has been doing electronic money transfers since 1871 and they were the first processor of both VISA® and MasterCard® bank-issued credit cards in 1976. First Data processed 12.2 billion transactions in North America alone in 2003. With a merchant account thru Carsdservice, there is no extra transaction costs involved.
Authorize.net is probably the most advertised electronic gateway. A lot of people think they can sign up with them and immediately start to accept credit cards. They do not release you need a merchant account as well. Authorize.net even uses the First Data platform to help facilitate transaction processing.
Verisign also has a payment gateway. Verisign is probably the most recognized name because they also offer other services and products for the internet (i.e. SSL certificates, domain registration, hosting, etc). You do not need to purchase everything from Verisign to have a successful e-commerce business.
All three gateways above will provide an API in which to capture & process credit card data on your secure webpage.
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Corey