Shipping
Of course, shipping couldn't be easy, now could it. The calculations for the USPS seem pretty simple: you just punch in a weight, and they have some formula for calculating the maximum allowable volume. Hell, it may even be constant.
But UPS has formulae that make the tax schedules look elegant. Basically, they calculate a weight that the box is supposed to be, based on volume, and the weight that the box actually is, and choose the higher one. So you get hammered hard on packing materials and double-boxing.
And you know how UPS domestic has like ground shipping where you just pay a minimal amount and let it take forever? Well overseas it's entirely impossible to distinguish one class of service from another. Near as I can tell it's just like random priority froofraw.
And then there's the old "insurance vs. customs" game. If you insure your package for lots of money, customs may make you pay that in full just to let it enter the country, effectively costing you the entire value you insured against loss. Lose-lose!