Merchant Account Integration solutions for a complete newbie

[Mavent]

My sad and dreadful story:  A company hired me to do web design for them.  I made the mistake of fixing a .php error on their site a month back, and I've somehow ended up replacing their "real" programmers.

 

They're paying me to much money for me to tell them "no", but they keep asking me for more and more complex code, and I'm nearing the end of my ability to fake my way through it.    Every time I tell them "I can't do this", they smile and say "Sure you can".  Then I have to run off and find a way to do it.

Today they want me to hook a merchant account up to a sales page.  I have literally never done this before. They literally said "Here's our eNom account. Make the page happen."  Does anyone know of any "off the shelf" solutions for hooking my .php order page up to a merchant account?  I'm not looking for "free", BTW.    I'm willing and able to buy a good solution.