I do not print packing slips for my sales, so I am assuming it is just the paperwork you include in the package for the customer. If you have charged the customer the wrong amount (too much postage), Just tell your customer that postage was less than originally estimated and refund them the difference. You could just cross out the postage cost on the packing slip and put in the correct amount by hand. I don't think you can do it any other way. I always use "calculated postage" on my listings, so it is always the correct amount.
