It's similar to other Shopping Cart Interfaces (SCI's) such as this one from Liberty Reserve
or this one from Pecunix
. It requires a basic knowledge of HTML forms, which does not really require any toolkit, be it PHP or Rail or anything else. I'm disappointed that mybitcoin chose to release software first without any documentation.
But if you can understand how LR/Pecunix's SCI works, you can easily use the same techniques with mybitcoin, obtaining the HTML form variables by playing around with mybitcoin's merchant tools -- with the "encrypt" feature disabled. Reading the PHP source helps, but I found the MBC 'merchant tools' link to be the most helpful resource.
Hopefully they will read this thread, and post some documentation