There's lots of things that could cause the change in price. Weight difference (scales used), dimensional weight (package size), residential vs. commercial address, etc. The price given on the website is just a quote based on the input you give it. Once all the variables are verified by Fedex they calculate your cost. They've been pretty fair every time I've used them, I find them to be cheaper than any other service for medium to large packages. Often the price changes by a buck or two but I don't sweat it as my cheap postage scale or bathroom scale I doubt is as accurate as they ones they use.