Shopify App Developer to fix broken Shopify App

We are a music publisher located in the United States. We previously hired a Shopify app developer to create for us a custom digital-download app that would automatically watermark digital files with the customer's information before delivering it to the customer. The app still functions, but unfortunately parts of it have broken making it harder for customers to find their digital files. Customers used to receive a separate email with their digital files, but now can only access their digital files with a small link in their order confirmation email. We would like customers to receive a separate email with their digital files like how the app originally functioned. Once that has been fixed, we would also like the digital files to be accessed straight from the "order complete" screen as well as in their account profile. We essentially want this custom digital download app to function identically to the base "Digital Downloads" Shopify plugin on the customer side.