You should point out to your buyers that listings with Simple Delivery do show a faster service.
If they choose the more expensive 'Express Tracked Delivery' they are paying for a 24 hour service.
I have chosen only to use Royal Mail so, buying from me, that would mean the buyer had selected Royal Mail Tracked 24.
If you buy your own label, you lose the postage cost as (unless you offer free delivery/seller pays) the buyer would be refunded for the unused label.
You would be paid. You could amend the tracking and insert your own or wait until 14 days after the transaction when payment would automatically be released.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)