LOL PayPal fees. So I took a bit of time to calculate the exact (to +/-$0.01, due to possible rounding difference in PayPal) fees for US and foreign payments. If anyone finds it useful for other transactions, the fees are as follows:
Fee for payment for goods to a US account coming from a US customer is 2.9% + $0.30 or Fee_domestic = ((0.029*initial amount) + $0.30)/0.971
Fee for payment for goods to a US account coming from foreign customer is 3.9% + $0.30 or Fee_foreign ~= ((0.039*initial amount) +$0.30)/0.961
(ninja informed me that the first batch of invoices was slightly undercharging, because 3% and 4% are actually not good approximations for correct PayPal fees. But it's been fixed now for the later invoices. You can see the values updated on the published spreadsheet.)