He f***ed you! Print this PDF, march down to the shop, and demand your money back.
https://www.mass.gov/doc/approved-firearm-safetylocking-devices-06-15-2020/download
M.G.L. c. 140, § 131K states that all sales of firearms and large capacity weapons in Massachusetts MUST include a safety device approved by the colonel of state police. This requirement applies to any seller including a wholesaler. The seller is not required to install the device so long as the weapon is accompanied by it. Failure to comply with this provision will constitute a breach of warranty and an unfair trade practice. This section does not apply to non-large capacity rifles and shotguns.
Sellers should ensure that the safety device is appropriate for the weapon being sold.