I've set up my return policy in the same way you have, buyer pays return postage, and the few times I've had someone use the return system for a change of mind reason, the system has automatically applied this rule and shown that a refund of the item price only will be issued. This is all vetoed if the buyer chooses a 'not as descibed' reason instead. (Change of mind reasons are any of the following "doesn't fit, changed my mind, found a better price, just didn't like it, ordered by mistake", while not as described reasons are "doesn't work or defective, doesn't match description or photos, wrong item sent, missing parts or pieces, arrived damaged, doesn't seem authentic").
eBay may apply EU distance selling regulations when someone in the EU initiates a return for any reason, as over there the buyer is entitled to a full refund - including original postage - no matter what the reason is for wanting to return. They remain responsible for return postage costs.
If none of these apply to your return request, there may be some other reason the system is trying to issue a full refund, and the only way I can think of around that is to implement a restocking fee, however this will apply to all returns (though it can be waived, some buyers will either be put off, and a small fraction will even use a "not as described" reason to avoid as much cost as possible). The restocking fee can also only be a maximum of 20% of the sale price, so for a $50 item that cost $22 to post, you would only recoup $10.
