Weird airgun delivery rules

USPS, FedEx, and UPS all have different rules insured price point delivery standards

Could be DOT rules. Airguns are typically not regulated DOT, where a 4500 PSI tank is considered a hazardous cargo due to risk associated with high pressure vessels. It's not because it is for airguns, but it comes under all the DOT rules for things like Oxygen, acetylene, nitrogen and all those other industrial, high pressure containers. I've never heard of an accident where an airgun being shipped hurt someone, but acetylene going off really has power.

Not sure if that's the case, but seems like the DOT would regulate pressure vessels, but not airguns.

I'm not sure for ground and express but for freight, if a shipment doesn't meet a certain weight for most hazardous materials, it wouldn't need a HAZMAT label.

I don't know what the limit is for UPS, but on FedEx, anything over $500 must be signed for unless the shipper (or payer if different) says otherwise. 

The regulatory environment will improve. The cavalry is on the way!