They suck. I work in high-volume duplicating and ship things out every day through a corporate account, and the company gets a negotiated discount. In the rare instances when I send something personal, I use a calibrated shipping department scale to judge the weight. I am quoted a certain amount for shipping, but when I look at my bank statement later, the amount is ALWAYS HIGHER. $7 for a derailleur going UPS Ground, after being quoted $4 something. We asked the UPS sales rep, and he was unable to explain the discrepancy. We asked the regional sales rep, and he has no explanation either.
I had a customer that shipped a bike through UPS, and he also got burned with the UPS surcharge. Seems like these after-the-fact charges are part of the UPS business plan.