This would be a useful feature in case a buyer requests a specific postage option or service that you didn't list, then it would be all included in the price and description instead of some ad-hoc after sales patch-up. I see no reason why adding postage options should not be allowed once bids have been received, I'm not talking about removing options.
Incidentally, I notice it is still possible to remove all payment options from an item that has bids, that seems a bit daft to me and could be open to abuse.