as far as I know, the 30% is completely different - it applies to visa card purchases in dollars. The customs people can charge you duty- which is often between 50% and 100% of what THEY think the item is worth. Actual tariff may only be 20% or 30%, but then they can charge you IVA on their estimated sales price, another 21%, and they dont think in terms of Black Friday sales. Then there are often other misc. fees. Average rule of thumb is that by the time you get a Porsche on the street, licensed, in Argentina, its double US retail. A quarter million for a Carrera. The new electric Porsches are averaging $200k out the door in the USA, thats a pretty expensive car here.
Sometimes having a receipt will set a price, sometimes not- its up to them. But they have never once done that to me, travelling on a US passport. They focus their ire on argentines, mostly. A foreigner can usually bring one or two of almost anything. A dozen Iphones, probably not. but 2 laptops, or, say, a home stereo, a sewing machine, pots and pans, new sheets, spices and condiments, art supplies, cameras, phones, monitors, printers, fender guitar amps, (all of which I have brought in without comment) have all been let in without comment.
When you have multiples, things that are pretty clearly for resale, thats when they get picky.