Convert late fees to default currency before comparing to the set minimum figure
At the moment, late fees can be worked out as a percentage, say 10%. A minimum figure can be set as well, say 10.
If the default currency is USD, a customer who has an invoice for $30 outstanding would have their late fee calculated as 10% of $30, which is $3. This is then compared to the late fee of $10, and the higher of the two figures is charged - $10.
Unless I've misunderstood how things work: If products are also available in JPY, the exchange rate is currently about 90 JPY to one USD. So their $30 package would mean that their invoice was billed at 2700 yen. The late fee is then calculated at 10% of this, which is 270 yen, which is then compared to the late fee of 10 yen. The higher of the two figures is charged - 270 yen.
That is equivalent to about $3, so that customer is only charged a late fee of $3. Had they been billed in USD, their late fee would have been $10. It seems to me that the minimum late fee in the settings screens should be a default-currency figure, and the actual late fees converted to default currency before they are compared to this minimum. The amount of late fee should not vary depending on the currency of the invoice.