Get inspired
You could build anything with Goaddon. Feel free to read through our project ideas. Some could be usefull for your ecommerce company, others could be offered to ecommerce companies by you as a service provider.
What do you get from Goaddon?
By subscribing to the Goexplore addon you get a website that online shops can use to sign up.
By subscribing to the Gofetch addon you enable the merchants to import their orders into your database.
If you install the explorer-at-goexplore repository, your website even get an orders page that you can add the relevant buttons to.
What do you need to build on top of it?
So by now you have a website where a shop can sign up and enable the recurring import of orders into your database.
You would need to create a web page that can be accessed by anyone, not only authenticated users. That page should contain a form where a customer can enter the relevant order number and submit their reason for starting the RMA.
To ensure only real customers get to start RMA's you could require them to confirm their identity by sending them a verification email.
Once an RMA has been started it should appear in an interface accessible for the merchant.
It may vary between industries or countries how a shop prefers to manage their RMA's. For example they might want to send pre-paid return papers to their customers, and for that you would probably need to integrate with different carriers in different countries.
How fast can you build it?
You could probably build a proof of concept version in 1-3 months. However, your approach from their could be more or less ambitious.