It's been about 7 years since I retired, but my past life was in international shipping... mostly out of the U.S. but did have some contact with the inbound services and customs procedures.... I'm sure by now many of them have changed and since it's been as long as it has, I've probably forgotten some of the steps and procedures.... the duty rates are set forth in the U.S. Code of regulations in the tariff of the TUSSA... I've forgotten the actual name of the acronym... it's a huge book with multiple columns and classifications for duty rates, and nearly impossible to read.
As I remember everything is subject to duties, but under some classifications, the duty rate can be zero... check with the broker you use and explain your products and maybe he can re-classify or at least ask the customs specialist in the U.S. Customs department for a re-classification.
Back when I was working, any order over $250 (commercial orders) were required to be entered by a licensed customs broker... Shipments under $250 can be cleared by the consignee without a broker... that could be changed now... rules were constantly being changed.
Another rule (when I was in the business) was that every shipment had to have a customs bond.... either a one time bond for that shipment, a single entry bond.... or a term bond which was sort of a revolving bond for all your shipments and usually good for a year... sometimes it was cheaper to get a term bond than to use the single entry...
Shipments from some countries were not subject to duties... i.e., when I was in the business, China was considered a "most favorable nation" and many shipments were not subject to duties... wearing apparel always had a duty rate, but other things may not.... I would think pen kits from China would fall under the not subject.. I may be wrong.
Shipments that come through the post office are subject to customs, but generally were auto cleared by the post office...any duties were subject and collected by the post office.
Any time they came by courier, (TNT is a courier company based in The Netherlands, I think... Fedex, DHL, MSAS and UPS are courier companies... DHL is owned by the German post, and I've forgotten who owns UPS... I seem to remember it was also sold some years back.... FEDEX is US company and based in Memphis...MSAS is the old McGregory Squire Air Service and it's also foreign but I forget whether British or Australian although I think it's Australian), the shipment will be subject to their courier charges, often paid by the shipper, and customs clearance and duties paid by the consignee.
As said, I did mostly exports shipping out of the country, but worked for a number of companies that did have import departments and even took the customs brokerage exam once myself and only missed by 5 points... it's an 8 hour exam and designed to pass less than 5% of the applicants at any one time.... the exam cost $300 when I took it, if you passed it cost you the $300.... if you failed to pass, you were refunded $200... I didn't pursue the license as I preferred the export side of the business.