I use the Plastic plug-in from 360Works to do credit card processing within FileMaker. It’s a daunting task at first, but once it is setup, it’s absolutely great. This guide explains how to setup a First Data credit card gateway API for use with FileMaker using the Plastic 2 plug-in.
From First Data you will first need to download the Web API from their site.
Inside the file you’ll find multiple certificates and passwords.
We need three files:
XXX.auth.txt
XXX.ks
XXX.ks.pw.txt

In XXX.auth.txt there is our username and password.
Use this in the CCProcessPayment function.
We’ll need to add XXX.ks to our database inside a container file.
XXX.ks.pw.txt Is used in the CCSetCertificate function.
FileMaker Functions
CCSetCertificate
CCSetCertificate ( _plastic::firstdata_cert ; "ks.pw.text_password" )
CCProcessPayment
CCProcessPayment ( "XXX.auth.txt_Username"; //Merchant Account User ID "XXX.auth.txt_Password"; //Password $amt ; //Charge Amount $cardnum ; //Card Number $exp_date ; //Card Expiration Date "verificationCode=" & $cw2 ; //Check Verification Code //Optional Information "chargeDescription=" & "Order " & INVOICES::Order Number; "invoiceNumber=" & INVOICES::Invoice ID; "firstName=" & Customer::B_FirstName; "lastName=" & Customer::B_LastName; "country=" & Customer::B_Country; "zip=" & Customer::B_PostalCode;