Hi, I have an app that provides Shopify sellers with a catalog of products that they can sell. Orders are drop-shipped as the orders are placed.
I now have a new requirement that some of the items in the catalog should allow the buyer to upload a photo and zoom/position it. How would one accomplish this? Each seller could be using their own theme, so how does an app automatically add a "Customize" button to the product page which redirects them to a page hosted in the app perhaps?
As you mentioned, each seller is using their own theme and therefore not all themes are the same. The typical solution that I end up seeing in these cases is offering the merchant a snippet of code to insert into their own theme file, with general instructions on how or where to place it, in addition to offering assistance if it's too much for the merchant to figure out (totally depends on their skill level and understanding).
An example of this in action is Shopify's free Product Reviews app if you want a reference.