I think a crack like that can be repaired, but you should also make your post office aware of it and begin a claim. You may be able to get partial money for the value lost due to the breakage and still get to keep your rifle. Sticking to the gun building aspect of things, If it were mine I would begin by removing the triggerguard pin and using super-thin CA glue. Carefully take the lock and triggerguard out and wax them so nothing sticks, then replace them to keep things in line. The glue runs like water except it has no surface tension and will flow into every crack possible. Feed the crack until it appears to not take up any more glue, then let it sit overnight. This should stabilize the wood and prevent further cracking, allowing you to remove the hardware and perhaps add some structure beneath the lock if it is needed. Without seeing more or feeling how loose the crack is I'm not sure what else I would do. Sorry that happened!
-Eric