I'm working on an e-commerce site and am a rails developer; due to the nature of the items, I am managing them in their own database so I'm really looking for something that is only for the shopping cart aspect (there are things like heavy nesting and integration with other pieces of the app that would be very difficult to reproduce).

It seems like there are two ways that I can go. (1) One way is rolling my own shopping cart and using something like Stripe (which I have been evaluating and am working fine with it). This literally could be as easy as creating an orders table and a line items table and a lot of front-end. (2) Or I could try to integrate into a third-party shopping cart like Shopify. I am not really sure if I can just use the shopify shopping cart or whether there is any advantage to this. If I already have most of my app done, would shopify (or another shopping cart app) provide any significant benefit (it clearly could)? Or would the integration be too much of a headache? Like for example, when a user 'adds to order' on my site, can I post to shopify and associate it with that user?

thx