Thank you for your message, I would be more than happy to offer you some ideas on this.
There are two third-party applications that I can think of off the top of my head which would give your customers this ability:
- Uploadery - $8.99 USD per month with a 14-day free trial.
- Upload Fields- Free Plan Available (Additional charges may apply)
Both options will allow your customers to upload files with their orders and in both applications, the files will be visible in your Shopify admin within the specific orders. I would recommend trying out both apps to see which one works best with your idea. If you have any specific questions about the apps, I would recommend reaching out to their dedicated developer support. You can find all application support information on our App stores listing pages for each app:
Alternatively, you could hire a Shopify Expert to build you some sort of customization, however, an app would most likely be the easiest and most cost-effective solution!
I hope this helps you out, please feel free to reach back out here with any further questions, we are always happy to chat.