My experiences in this area on ST have been very positive, although it is always a slightly different process each time. I am E+ on ST and often travel with excess baggage for my job, either pieces or weight or both, and I rarely pay a charge. First, it depends greatly on which airline you are starting with and checking in with...if they allow it for their own customers, then the bag will be checked through to final destination no matter which airline follows, including out of network airlines. If you are starting with an ST member that does not have that policy but are connecting to one that does, it usually is just a bit of friendly bartering at the check-in desk, but keep it very friendly! because that first agent can kill the deal if he/she gets their back up. Have a good funny story ready...like your spouse found an absolutely wonderful cast iron lawn ornament and you just had to bring it back. Counter bartering often (but not always) works even with out of network carriers if you are connecting to a TATL flight that has different rules than the local domestic carrier. If it is only one of the two bags that are overweight, or a third bag that is much smaller than the primary two, that helps. Any leg in F will help tremendously, even if it is just an upgrade, especially in developing nations with inexperienced agents. It is slightly more effective to come at peak checkin time to add a bit of pressure to the agent, but it leaves you less time for a backup plan (one airline forced me to check a bag as cargo at the last minute, but the cargo office was across town, and the cargo goes on the same *@#$* aircraft, go figure). But if you stay cheerful, be persistent, and act like you have done it this way a hundred times, it usually works.