eBay are just trying to mitigate loss. We also send various items using standard letter and to be honest it's far more reliable and faster than parcel post.

We adopted a trick by adding a QR code sticker in the top left corner. This is the zone senders can use for any purpose, return address, advertising, anything you like. It gives the impression of a tracked item to the receiver and reduces the threat of a fake NDR claim.

When you scan the QR code it brings up some information including a number, similar to a registered item. We change the postage in our listing to Standard Parcel Delivery - Registered, which is not a tracked service.

We got the same email and ignored it. Your items might be put lower in the BIG FAT ALGORITHM using Best Match sort, but you will be in the same position as Price + postage: lowest first, sort, which is probably what the majority of buyers use.

eBay is a discount store and people shop here looking for the lowest price.