LM state that they cannot guarantee delivery if you use First Clas post, which to me puts the risk on the purchaser's shoulders, not the vendor's shoulders. If the purchaser wants to be guaranteed delivery, choose an appropriate postal service. I fail to see why it is LMs fault that the OP chooses to use a (cheaper) delivery method that does not guarantee delivery. That is a risk inherent that has been accepted by the OP by choosing that method, presumably after conducting their own risk assessment.
I also don't see how a charge back would necessarily be successful, for the very same reasons. Just because the ticket was not received does not mean that they charge is not a valid one. LM have sent the tickets which is all they said they would do, that RM have failed to deliver is NOT LMs problem - once in the hands of RM, there is NOTHING LM can do about what happens.
All that said however, it does seem rather silly in this day and age for LM to even offer a system of delivery that is not guaranteed.