Your Post Office should still have accepted the parcel, if the label had ' Marketplace' written on it. Under Simple Delivery, should the postage label received not cover the full cost of the parcel, eBay pay the difference.
It essentially means the post office should accept the package even if the label doesn't perfectly match the item's actual weight or size, and any issues will be resolved between eBay and the carrier.
If your Post Office did not accept the parcel, they were the ones in error here.
@comicseller42