Thanks to all who have replied.  Don't think cancelling a sale will make a happy customer - and they may not understand the reason either.  And may not re-purchase!  And can't remember if cancelling results in a defect - need to swot up on those things again.

The item was over the weight not size.  I just underestimated the amount of packaging.  And I can't package before a sale - just a waste of time cos it may not sell and I may end up giving it away.  And as it's been relisted several times I really can't remember eBay's suggested label as I could have listed it several months previously!

And I really don't want to pay for a more expensive label then discover I've wasted my money cos it was lighter.

Really a bit of a pickle!  Have managed so far with Simples Delivery until now.  Since I don't seem to be able to change the label after purchase I will have to pack up the dodgy ones prior to listing.  What a nuisance!