Chopping a cart

I have my own home-rolled content management system. I want to use my CMS as a content generator for an already existing cart.
If you look at the page-content for most carts, each dynamically generated product choice division is little more than a mini-form that sends a procuct-ID (and perhaps a few other such key-numbers) to a session-oriented back end.

Some existing e-commerce systems must be easier than others to cut apart, so different content-generation modules can be swapped in an out. In other words, if the backend transaction POST processing advertises a well-known API, then you could use any conforming content front end you might want.

In other words I want to throw away the content generation portions of an existing cart, but retain the POST/transaction processing.

Does any of this make sense? Are there any particular systems you might recommend, for this context? I am a PHP and Java developer. Any feedback on OpenCart?