On a theoretcial and practical basis (which I say because most of my stuff goes as a letter, which don't attract RTS postage fees), I would personally consider such returns to come under the same store policies as "change of mind" returns, albeit an unauthorised return.
The most common practice when dealing with change of mind is refund less expenses incurred, so usually that's item price but not postage, and in some cases a restocking fee - I would personally consider implementing a restocking fee if I sent a lot of parcels to officially cover these kinds of things, and then just waive it if no additional costs are incurred after outgoing postage, but apply it if I get hit with a return charge, or there is some damage / lost of value to the item due to something the buyer had done.
If you want the best chance of getting the buyer to reply, I'd message them with something like "if I am not able to confirm your preference by X date, a refund of X amount will be automatically issued", where X is item price minus the $15.00
If they don't reply, they can't blame you for taking that action, if they do reply, you get the chance to reach a mutually agreeable solution.
