AP advertise that as long as both parties reside in the Express Post network area, they guarantee that the item will be delivered the next working day, and if not, the cost of postage is refunded. It's easy to check before sending as the available postcodes are specified on the back of the satchel / envelope, and if not, they're available on separate documentation inside the PO and on-line. If either party resides outside the designated area, there's no point paying the extra cost for Express Post.

I've lost count of the number of Express Post items I've sent over the years, but in all that time I've only ever had to claim for reimbursement of the cost of postage on one occasion, when the item wasn't delivered until 2 days after having been posted, and as far as I'm concerned that's a pretty good track record.

Incidentally, if you offer Express Post as an option, it's a simple matter to transpose the list of available postcodes from the AP site into your listing so that buyers will immediately know if it's worth paying the extra for Express Post, and if you as a seller live outside the network area, there's no point offering it at all.