Hans,
Sorry to here that.
I had the same "hiccup" with OPT.
At first I suspected a "bait & switch" being the borderline paranoid type. Not so I discovered.
Apparently some of the people that enter shipping info into their website are/were entering it from the manufacturers specifications for the products. That is: without OEM packaging or shipping boxes.
Judging from the USPS, FedEx(Boooo) and UPS websites, even a few tens of grammes can push the rate up by 20-40%
OPT split the excess 50/50 with me so I can't grumble too much.
They did, however, ask me about 20 times if it was OK to use my CC, so perhaps I'm the dubious beneficiary of your dilemma!