For me this glosses over on why would see the same 100% markup on the customs duties as they rise. A 100% at a low tariff rate is just pricing in the increased paperwork and accounting, but at a super-high tarrif rate this become pretty unjustifiable. Why is not more likely customs becomes mostly a passthrough cost?
Until you get to extremely high-end goods this multiplier system works fairly well to accommodate the various costs the business has. It is an assumption that there are many business costs that scale linearly with sale price. In reality not all do, but there are many: insurance, return costs, loss, and customer service expectations all scale with dollar values.
Edit: if we switched from an import tariff to a foreign goods sales tax we could avoid this particular problem.