For the tandem I just shipped, they didn't weigh it at all. Bikes go by the equivalent of dimensional weight. The baggage lady at PHL said that bikes get charged like they are 100 pounds, and tandems like they are 150 (or something like that). So, I would assume that weight is not really an issue.

As an aside, to ship my fully assembled tandem from Philly to Minneapolis cost around $125, which included buying the boxes ($30 for two), insurance (~$15) and shipping. I dropped it off on Sunday and it was there on Tuesday morning. Actual shipping cost was around $80 or so. This was for an unattended bike shipment, like a FedEx or UPS shipment. They will hold the bike for two days on the other end for pick up.