Now this doesn't help anyone now, but how I tend to deal with postage is to buy in bulk. It doesn't work with fixed value stamps or parcels, but in order to keep down the day to day sending a letter costs, a first class stamp is always a first class stamp regardless how much you paid for it and how much they are now. I buy business rolls of stamps so although postage is now I think 41p my stamps were 27p and are still valid. Might be of use to someone.
Make sure you price postage accurately for eBay sales, don't guess at it then find out later you should have charged more, weigh items and put the details in a price calculator.
And try lighter packaging. Boxes may not be as necessary as you think, wrapping things in a number of layers of bubble packing and then some parcel tape is often quite sufficient. Businesses send almost everything in plastic wrap these days - that's why!