Any tool for watermarks for digital content on shoplo?

Hi everyone!

I'm looking for some tool to add watermark for every e-book sold on shoplo (it's like shopify). There is some API in shoplo (e.g. webhook after payment) and I'm looking for some tool to integrate it with shoplo - any ideas are more than welcome!

I'm a senior developer but I have no idea about watermarks, e-books, e-commerce and automation (I can create my own script but I'm looking for some automation).