[mothered]

What should I do in situations in which the agent asks for the product back. What then?

In this case, you (seemingly) send the product via an untracked service, for example by post.

Post provides tracking (a little more expensive) and non-tracking whereby when using the latter, If the package Is lost In transit, It's whereabouts cannot be Identified hence the Item Is lost. As such as far as you're concerned, you did the right thing and subsequently as far as the merchant Is concerned, they have to do the right thing by you- the customer, and take the next step In fulfilling your request.