Dimensions: 85 mm x 56 mm
Area: 4760 mm^2 or 7.38 in^2
Copper: 4 x 1oz layers
Copper Weight: 0.205 oz = 0.013lb
Copper price: 0.013 * $4.50/lb = $0.0585
And that doesn't include the copper removed by etching. So if they paid a 6c tariff on each raspberry pi board, they'd be overpaying.
Can they generate a certificate claiming each board contains no more than this amount of copper, overpay the tariff by a few pennies, and carry on?
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.