Honestly...you gotta pack it just exactly right, and present it to the clerk in just the right position so they measure it the way you want them to. When they measure, you have to stay on top of them and sometimes point out that the place they are measuring is not indicative of the actual dimensions of the package.
Clerk to clerk can make a HUGE difference in how something is measured, and an inch here or there can double or half the price.
Shipping is an art form!