Problem here is Mailboxes Etc. employee does not know what constitutes a firearm and what does not.
Never dealt with Mailboxes, but I can tell you that some USPS employees and UPS employees don't know their employers' policies related to guns. Sometimes it seems to be sort of luck of the draw whether you get someone who knows what they are doing. I had to try three times to get a USPS worker who knew that it is legal to mail a (modern cartridge) shotgun to a FFL in another state.