You can customize the return address for each of your stores in the Addresses Panel on your Store settings.
Click the Edit Addresses button, uncheck the option "Same as origin address" under the Return section if you want separate return and destination address then enter in the store's return address.
Amazon Buy Shipping Return Address Caveat
When using Amazon Buy Shipping, Amazon will not allow SKULabs or any other product to list separate origin and return addresses.
SKULabs recommends creating a case even if it goes nowhere requesting the following of Amazon.
"My local post office is giving me trouble accepting packages because the Ship From ZIP code does not match their post code. I need to have my returns go to one address and my shipments originate from another address in order to resolve this. Using Amazon Buy Shipping I can only specify the ShipFrom address and am unable to override the return address to be a separate address. Can you please help me resolve this issue with my local post office? They don't get credit for the packages I ship using them when the labels are created incorrectly like this."