My understanding is that with Simple Delivery the Post Office are supposed to accept any parcels below the maxium size 61 x 46 x 46cm and to that end there should be no size / weight printed on the label, just "Marketplace Seller" printed in the bottom right hand corner.

Over the last couple of months there have been several mentions of labels that do show at least a format, possibly also a weight and the common thread is that these are labels where Ebay have provided a QR code and the label has been printed at the Post Office. It may be that the Post Office are still supposed to accept these parcels but you can see how they may not if presented with a small parcel and a label that says 'large letter.

This was a thread this week mentioning the issue and a refusal to accept the parcel.
https://community.ebay.co.uk/t5/Seller-Central/NO-LONGER-SELLING-DUE-TO-SIMPLE-DELIVERY/m-p/7877017#...

And there have been several others, here are a couple I turned up through search
https://community.ebay.co.uk/t5/Postage/Simple-delivery/m-p/7836222
https://community.ebay.co.uk/t5/Seller-Central/Simple-Delivery-Nightmare/m-p/7847744

I wondered if you could investigate what's going on here with these QR code generated labels please?