Lisa here from the Social Care Team here at Shopify.
Beautiful prints you have on your site. If you wanted to set up a fully custom & approval design process there are some programs to help streamline this process.
For the process of going back and forth for approval and adding comments, there are services such as Go Proof available. This service will allow your customer to preview the work and add comments at their leisure. Instead of sifting through your emails, each account is separate and sorts into 'completed' work.
If you haven't come across it yet, I'll let you know about Shopify's 'Digital Downloads'. This app will allow your customers to pay you a deposit for the work they'll receive, then you can invoice them the final amount upon completion using the draft orders function of your admin. Here's some additional information about selling a service on Shopify. How this would work in your situation, is the customer would commission you via the digital download on your front page. You could use Go Proof to make the edits. Upon completion, you can upload the digital download to the app, and attach this new custom product to the invoice. When the customer completes the payment, they will have access to the file, and you can send them the print as well! The physical product can be labeled as a 'blank custom' in the weight that the art is, this way you can still use Shopify Shipping to print the labels and post the final piece. I hope this helps you manage the approval process more efficiently.
Feel free to reply if you have any questions on the suggestions.
if you're still looking for an app like this, I recently created one that does exactly this. It's called Proofer Proof Management: https://apps.shopify.com/proofer
It automates the entire proof process flow:
1. You select an order from your Shopify admin backend
2. You upload a proof file and add an optional note for the customer
3. The customer automatically receives a notification that their proof is ready to be reviewed
4. They can view the proof on a new sub-page of your store (similar to an order status tracking page)
5. The customer can either approve the proof or request changes by entering a comment
6. You receive the approval/rejection notification and can fulfill the order or respond with a new, revised proof
On the app's order dashboard, you're always able to see which orders still require a proof to be uploaded, whether the customer has accepted/rejected a proof, etc. The app can also automatically remind customers if they haven't yet reviewed the proof after X days.
If you decide to give the app a try and have any questions or feedback, just let me know, I'm happy to help!