Hello @heatherw1,
You'll have to contact the Technical Support of the Shopify app so someone can help you manage the returned items from your customers.
But if you're looking for an app integrated with QuickBooks to manage your store credit, you can find one from the Apps store.
In case you need some articles in managing your customers and sales transaction, here's an article you can read for reference: Income and Expenses for QuickBooks Online.
If there's anything else that I can help you with, let me know in the comments. Always around happy to help.
