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

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Where does it state that?

Ebay does reserve the right to charge extra for under declarations where the seller has selected a size other than that recommended by eBay:

'c) Under declarations.

If you amend the predicted dimensions and weight of your item at the point of listing, and the item is found to be underdeclared, eBay reserves the right to charge you for the under declaration as well as for any applicable administration or other fees.'

However, I'm yet to see anyone posting regarding this actually happening.  The only way eBay would know of any size discrepancy is if Royal Mail informed them.

Due to the way business contracts are structured and the fact that they are usually on a flat rate basis it is highly unlikely that Royal Mail will be concerned with the specifics of any individual item sent.  They will only be interested in the overall profile of all items sent, actual versus projected, which is normally then taken into consideration when the contract is next negotiated, usually on the anniversary of the contract, but can also be used to determine any interim changes to the contract price that Royal Mail deems necessary.