The T&Cs clearly state that you can still be surcharged by Royal Mail
for overweight SD parcels.

I had a listing with Seller Pays Tracked 48 , which does not require me
to pre-specify the weight. EBAY retrospectively changed it to SD up to
2kg without informing me. When sold and packed is was 2.2kg - so I
cancelled the SD label and bought the correct label via Royal Mail Click
& Drop. EBAY has after 2 weeks refunded the unused label costs to me.
Complex and stupid.