Yes, that's how the math usually works with properly-functioning global trade. Now prove that your math is correct. Can you hand over paperwork proving that it

Yes, that's how the math usually works with properly-functioning global trade. Now prove that your math is correct. Can you hand over paperwork proving that it is indeed a 4-layer PCB and not a 32-layer one? Can you prove the 1oz copper isn't secretly 2oz? Can you prove it isn't a copper-core PCB? For all we know that PCB is a 1.6mm-thick solid chunk of copper! And what about all the parts on it? Do the manufacturers of all the components on top of that PCB provide an exact per-element writeup? How many grams of copper are in that power inductor, or the ethernet jack's magnetics? We're still not entirely convinced your paperwork checks out. Could you please have a testing lab run it through a mass spectrometer, just to remove any doubt? Yes, we know it's a $1.50 board. No, we don't care. Yes, you really have to do it again for the next one-off shipment - you didn't go through the proper year-long type approval process, after all.