How much will I now pay in duties to ship a stylus from Japan?

It was shipped to me.

AFTER I received it, about a week later, FedEx mailed me a separate invoice for:

1. 15% duty (applied only to the net cost of the item, not applied to the add-on bank transfer fee, bidding fee, in-country shipping fees or shipping to USA costs.

2. Disbursement fee, $4.50 that FedEx collects for themselves.

total amounted in that case to +19%

I have read that sometimes they ring your bell and ask for you to pay the extra charges on the spot, in my case they delivered it and are trusting me to pay their bill.

That’s also exactly what happened to me recently, buying a used cartridge from Netherlands, via FedEx (actually, they auto-charged my card on file). If you’re buying new product from a manufacturer or scrupulous retailer, they might include tariffs in the total price and pay it up front. But when buying used - expect the above sequence.

If you are purchasing on Ebay, they have a SpeedPak option where they calculate the shipping and fees up front and add it to the cost of goods. You just pay whatever the total invoice is when completing your order and wait for it to be delivered. Tracking is included, though I'm not sure if there is a total value limit on the item you're purchasing.

A tariff is not paid by the seller in the country of origin, unless the seller chooses to reduce the cost of the item so as to absorb the tariff. The tariff is paid by the buyer in the destination country. By definition. “Duty” = tariff in this case.

FedEx Invoice billed: 

Disbursement Fee ......... $4.50

Customs Duty .................$16.35 (on net cost of cartridge Y15,600 ($109.) (Japan is 15%)

as I said, there is no duty collected on the various fees and shipping costs.

https://www.cbp.gov/travel/international-visitors/know-before-you-visit/customs-duty-information