Your question is worded a little odd, so I will prevaricate a bit on this one.
If the price and shipping were agreed to in the auction, you bought it and there are no variances between what was agreed to and the current circumstances then you have a contract and they cannot ask for more.
However if something has happened such that the payment is not as per the contract (like eBay has a glitch so the total given to you was not the total per the auction) or you have changed the contract conditions (eg you are now asking to have it shipped a different method, to a different place or you want work done to it before shipping, etc) then they could ask for more.
I hope that all makes sense.