Was it just over the relevant weight band for the selected label size or actually over the maximum weight permitted by the carrier for the service (15Kg for Evri and 20Kg for Royal Mail) and did you use eBay’s recommended size when listing or select your own?

Ebay states in their terms that if you use their recommended parcel size when listing that there won't be any additional charges if the item exceeds the weight and dimensions of the label.  The same should also apply when eBay provides a combined postage label.  If, however, you select a different parcel size when listing then eBay does reserve the right to charge extra if the item is found to be underdeclared. 

So provided you use the parcel size recommended by eBay, and it doesn't exceed the overall maximum weight and dimensions permitted by the carrier for the service, then there shouldn't really be any need to cancel the sale and relist. 

Ebay does also sometimes provide the option to adjust the label size before  generating the label.  Only seems to be available on certain devices/platforms though, such as the Android mobile website.

There definitely shouldn't be any need to actually package items before listing.  You really just need to weigh and measure the item and see if it is anywhere near any of the cut off points.

Out of interest, which carrier was it?