1 in 10 large letters going astray is super extreme. I'd suggest you might need to enlist the help of AP to figure out what the problem is there.
We use large letter untracked for 99.9% of our stuff and our reported loss rate would be somewhere near 1 in 1000.
We ask the buyer to validate their address and check at their local PO before sending out a replacement item.
Then we add a simple followup comment to the feedback we left for them that says something like
<UID > Replacement item sent for item reported lost in post <date>
We include their user ID so that other sellers can see their name too otherwise EBay scramble the name. That way if they are repeat offenders everyone can easily see it on their feedback history.