You can understand return rate percentage by using the Order Export feature. This method computes return rate based on the amount captured on the authorization.
In your Orders dashboard, filter for the desired time range and include both of the following statuses to only include closed trials:
Complete and Captured
Completed
Click on the Export icon in the top-right corner and export the .csv.
You can use the following columns to understand return rate on trials, through the formula provided below:
Originating Order Total = initial total order value, inclusive of any buy now items
Ultimate Total Paid Amount = the ultimate net payment from the shopper for an order, net of returns and including both buy now and TryNow items
Note: If you are looking for return rate percentage based on units returned, then you will need to either use an export from Shopify or from your returns platform and filter specifically for TryNow orders, either through tag or by exporting a list of TryNow orders from Merchant Portal.
However, the return rate based on units returned vs. dollars returned should be comparable.