Set the charges for each individual paper size.
NOTE - You should always have a custom size charge in the charges list. If the user specifies a custom size, Printer Squirrel will calculate the size from the height and width information. You need to specify the charge per square meter of paper, the general formula is (charge for a A4 or letter sheet) * 16.
eg: If you plan to charge $1 for each A4/letter sheet, set the 'custom size' charge to $16 (625 square cm(A4 size) * 16 = 1 square meter).
The custom size charge rate is also used if you do not have a corresponding paper size charge rate for the given paper size. It is always adviseable to set charge rates for your most commonly used paper sizes.