He is a legitimate doctor and hat is his practice address.
Unfortunately for you, you are wasting your time reporting it to the FBI, Secret Service, Postal Inspectors or any other federal agency.
First, you two engaged in a business transaction that went south. In addition, I doubt you have any written terms defining such. You sent the parts williningly meaning no crime occurred. This by all accounts it’s a civil matter.
Sounds like Payapal was used and that means you screwed yourself- as it violates their terms of services and means they have civil recourse against you if they want (not that they will). If PayPal was made aware this is firearms related, they will not refund any money and shut down both accounts.
No federal agency or US Attorney will even entertain this. Sorry you got screwed.
Your best bet is just go file in small claims court where you live and have him served in Florida. If he shows- judge will decide. If not, default judgement! Notify the Florida Medical Bird and place a lien on his cars/house and medical practice. Eventually you will get paid.