You may want to check that info. There would be no reason (from a application development perspective) for each customer to "develop" their own API. An API is a application program interface and usually 1 sided. A methodology combined with syntactical layout that allows you to draw out or place information in 3rd party system.
Using a RESTful approach is the easiest, safest and best methodology IMO and can be set up in days.
Even the hard ones, (aka PayEezy) only take a months. (FWIW I just did an interface with PayEezy aka First Data's API) by myself in a couple months. If it's going to take you years you might as well reevaluate another option.