Ill cover the merchant account / Paypal account. (Some versions of) Paypal
is an IPSP (Internet payment service provider). If you are in the United
States, you might even qualify for their web pro plan.
Or getting a merchant account (along with an electronic payment gateway
(LinkPoint, Payflow (owned by Paypal), Authorize.net/Cybersource) will help
you accept credit cards online securely. Most will give you an API to help
maintain consistency throughout the checkout process.
http://www.totalbusiness.com/Default.aspx?tabid=5527 might also help