I need an online shop with a custom interface (customizing items with Ajax, with a preview included). Writing it myself does not make too much sense (implementing all the payment options etc.), so I would like to use an existing online shop (OpenSource).I would like to build my own UI which, for example, tells the shop to add an item to its cart -- i.e. without using the online shop's native UI. More precisely, it should be an online gallery where the user can directly order an image if he likes it. The final checkout/payment page can be native again.Is there a shop system that supports this? Or is it still faster to write it on my own? Or are there better options?Additional informationI have written a JavaScript image gallery. The idea is that when the visitor finds an image he likes, he can then click some buy now button and is then shown a new page where he can see how the picture will look like (different sizes, borders, etc.).Finally the image goes to the shopping cart and the user can proceed to checkout. This is where I would like the online shop system to kick in. Not before, i.e. not in the previous paragraph.UpdateIn the meantime, I have found another system that really only provides the API: Moltin. Just in case somebody else is looking for a shopping API.
Searching for an online shop accessible via API
api;shopping cart
I would start with an existing platform like OpenCart or any other open source framework and THEN customize it to meet your needs.  Definitely no reason to do all of the hardwork that's already been done, but definitely worth customizing the remaining items on your to-do list.