Parcel lockers are for parcels and tracked items it is not compatible with non-tracked letters, unfortunately, there is not yet a flag on eBay that alerts the buyer that parcel locker addresses are not compatible with the letter rate service they have selected, which is not a great buyer experience for eBay buyers as their letter will be returned unclaimed.  So the only workaround now is to send a message to the buyer requesting an alternative address or wearing the extra expense in these categories you could be out of pocket on the sale.