What was the reason for the return? If for item not as described you should open 2 cases if these were separate items.
The seller would have paid for the tracked second postage through the second case, but as you have paid for this, enter those tracking numbers into the case details, escalate on day 4 if not refunded beforehand.
Unfortunately, there's nothing to enforce the seller to reimburse you for the postage paid.
@reg7025