For a single batch of imported goods with a customs value of NT$2,000 or less, customs duty, business tax and commodity tax are waived (tobacco, alcohol and tariff-quota agricultural products excepted); once it's over, the whole batch is taxed, not just the amount over the line.
There's another rule that's easy to miss: per recipient, within a half-year (Jan 1–Jun 30, Jul 1–Dec 31), the duty-free exemption is capped at 6 times; from the 7th onward it's taxed regardless of value, and the count resets on Jan 1 and Jul 1. Customs value = product price (after discount) + international freight + insurance and other costs.
Customs doesn't open and inspect every item — it screens by declared value, product description, the recipient's history, and random checks. An unusually low declared value, or one recipient importing heavily in a short period, raises the odds of being checked. Under-declaring saves a little now but costs more if you're caught and hit with back-tax plus a penalty.
Jumping Freight declares properly and to the actual value, and works out your count and the best way to combine or split first, so you don't get chased for back-tax afterward.
Sources: Ministry of Finance tax portal; Keelung Customs, Customs Administration, Ministry of Finance. Final duties and taxes are as assessed by Customs.