If you lie about the package containing a gun and it gets lost, stolen, or damaged a shipper will not honor any insurance.
When you sign the shipping document you're signing a legal contract. Lie and you've defrauded the shipper.
That means the contract and insurance are invalid.
People often claim a package contains "parts" or "tools" but when shipping a rifle or shotgun you don't have to ship it the fastest way like they require a pistol to be sent, so it's cheaper anyway, why lie and take a chance.
Note that UPS Stores will NOT ship guns or knives. You have to take those to a UPS depot or have the package picked up by a UPS driver. The paperwork for this are on the UPS site.
As above you can mail a long gun through the USPS but NOT a pistol.
Regulations state that the package CANNOT say "GUN" or FIREARM on the outside. At one time years ago they required that info be on the package but thefts skyrocketed so they now say no markings on the outside.
As above if it's going outside the state it has to go to an FFL Dealer. In-state you can ship it, but many shippers will refuse.