I don't know about travel insurance (always best to check with whatever you choose)... but I wouldn't put/leave anything valuable in a bag left for pick-up...only things that can be replaced, like clothes, bag, towel, etc....money, electronics, passport, etc should be with you, jewelry (minus wedding ring) should be at home, etc....of that the only thing (possibly two things) of any cost to replace would be the pack and the sleeping bag, and I bet those would not rise above any deductible amount on insurance. alternatively, you could get a cheap lightweight duffel type sack, empty your sleeping bag, etc into there to send by correos, and wear the pack. then it really decreases possible loss, and the likelihood it looks attractive enough to steal
edit: I see two bonuses to using a cheap bag: one, if it's plastic it may cut down on any possible contamination of bed bugs (though I think that is not likely anyway); two, as you walk easily along with your pack, now weighing a pound or two filled only with your jacket and lunch, you will look like a superwoman.