Lots of opinions here but no direct users? I have personally used shipbikes 3 times in the past year. It works great. Basically from pickup to delivery FedEx handles everything. You are just billing to a shipbikes account number to get a great discount; everything else id dependable FedEx. I work for FedEx and have a fantastic personal discount but I got an even better rate with shipbikes. I have also shipped two tandems myself directly via FedEx for less tan a hundred bucks each (with my personal discount). If you ship a tandem you have to keep dimensions to the smallest possible for the box as you will be bumping right up against the max size limits allowed.
Shipbikes will be your best deal. When you pack, remember your rate will be more determined by volume than weight so getting the box an inch thinner saves you a lot if money.