Firstly, make sure you choose the right size/weight option when listing (or if it's in your favour, use the eBay 'recommended' size).
As long as you have done this, you should have no need to pay any additional postage.

I have on a couple of occasions used the 'recommended' size/weight given by eBay, even though the item was borderline. after packaging the item went up to the next size band, but as eBay had 'recommended' the size, I just used the label provided. Everything was fine, no surcharge for myself or the buyer.