Fasttech will no doubt not add tariff charge to anything. It is up to the importer to pay tariff. When individuals order, it is up to the shipper to collect the tariff.
USPS doesn't seem to bother with them at least when the customs form on the package is low value and sent via USPS, they just deliver it.
FedEx, DHL, etc...Pay the fees and collect them from receiver on small shipments and also add their own customs fee. Sometimes they collect when delivering the item or they will send you a bill for the customs clearance and tariff.
The United States Postal service for Dutiable mail is supposed to collect a $5.50 processing fee, the duty due and a $6 handling fee per package on delivery. This only applies to mail determined to require payment of duties of tariffs. So if it made it thru customs without the an orange Treasury department envelope attached and got delivered to you....You are home free on the tariff
Postal Service Process for Handling Customs Duties and Fees | USPS Office of Inspector General
712 Customs Clearance and Delivery Fee | Postal Explorer
712.11 Collecting Postal Service Fees
Post Office facilities must collect a Postal Service fee from the addressee for each item on which customs duty or Internal Revenue tax is collected. The Postal Service fee is authorized by international postal agreements to reimburse the Postal Service for costs it incurs clearing items through customs and collecting customs duties at the time of delivery.
712.12 Collecting Postal Service Fee for Each Package
The Postal Service collects the fee on each package bearing CBP Form 3419ALT. When a single mail entry form covers several packages, the Postal Service collects the fee only on the package bearing CBP Form 3419ALT.
The CBP Form 3419ALT is enclosed in an orange adhesive-backed Treasury Department envelope attached to the outside wrapper or envelope. The form indicates the tariff item number, rate of duty, and amount of duty to be collected plus a customs processing fee, which is assessed on all dutiable mail shipments.