this could be your financial institution or an online payment processor.
there are way to many to mention, but most of my clients prefer paypal for its ease of use, fast setup, simplicity, and 3rd party consumer protection. It is fast becoming the standard for online payments.
Paypal has an instant payment notification (IPN API) which makes it ideal for creating an entire ecommerce application for automated operations, payment confirmations, notifications, and delivery methods ... albiet a daunting but worthwhile task in and of itself.
- fast and easy setup (signup, confirm your identity, address, bank account, and you're golden)
- fast and easy payments
- does not require your customers to sign up for an account
- does not require your customer give you their financial credentials
- requires no SSL security certificate purchase
- instant payment notification ability
- requires no software on your server, just a small block of code for the buy buttons, although you certainly have the option of using their api's and code examples to automate your sales and database data and payment confirmations. For instance if you are selling How-To E-Books, after the payment and confirmation through their IPN API, you can then deliver said product with a forced download protected by php, with the determining factor being an entry in their account database that they have paid and the payment is confirmed through the IPN API.
- higher per-transaction cost than traditional credit card payment processing
- high level of payment disputes from fraudulent customers, so be sure its set up right and you have a method of delivery confirmation, like a 3rd party digital product download site that works with paypal for delivery confirmation purposes.