Well your pal is wrong, depending on the service some mail goes by air some by road,
I've had an air gun sent RMSD refused by the CAA, so it was sent by road, not only did I get an apology for the delay I also got the cost refunded,
but I was very lucky that the buyer phoned me as soon as the postie passed his house & I phoned RM immediately gave them the ref number told them it was an airgun so it was sent on by road before it was diverted to Belfast.