I need help figuring out why my shipping charge is 3-4 dollars higher than usps.com rate quotes. No matter how I change around shipping/packaging section tare weights and variables I cannot get within atleast the dollar range in accuracy for the usps shipping module priority mail. all packages are set up to ship for the same price since I only sell 4 products on my website, all ship in the same size box. Its set up really simple, but I cannot get the atleast within the dollar range of accuracy, its off 3-4 dollars.