Seat belt retractors/pretensioners are required by law to ship with Class 9 Hazmat shipping, also known as Dangerous Goods. Because of the igniters that are used in the seat belt pretensioners. Just like airbags are required to be shipped with Hazmat shipping with UPS or FedEx. However, there are some seat belts that don't have igniter portion and are just regular seat belts, then they are non-hazmat. The non pretensioner seat belts are usually used for rear seats of the vehicle.
(USPS) United States Postal Service does not offer Hazmat shipping for seat belts/airbags because they are Class 9 explosive safety devices.
This can be found in Title 49 of the Code of Federal Regulations (49 CFR, Parts 171-180), can not be ignored. Here is a direct link to DOT regarding seat belt hazmat shipping