You cannot protect yourself for any item sent if there is no tracking or a 16 digit number on the proof of posting for those numbers to show delivery. Of course, a large letter size does not have this 16 digit code, and extra does have to be paid for tracking.
I cannot see any items you are selling which are a large letter size they would all be a small parcel and if under £20 in value, no extra tracking is necessary, you get a 16 digit number on your proof of posting. It's not exactly a tracking number, but will show if an item has been delivered or not, all you need for any case for item not received. It insures an item up to the value of £20 for loss or damage too.
Selling small items of low value in large letter size, perhaps group these together to make them into a small parcel size,? Then, you'll have that 16 digit code.
@littlewedding