That's because you are using Express Post satchels, Beryl, and they don't qualify for Paypal's Seller Protection. However, you are not restricted to using only prepaid satchels for Express Post. You can also provide your own packaging, in which case you will be required to complete a lodgement form which is stamped by the PO, and because you now have the documented name and address of the buyer, you now qualify for Paypal's Seller Protection, exactly the same as you would for any other parcel which is lodged over the counter.