Rate Exchange Fee with low value currencies

Hi. I asked a question on the support page for the plugin at WP and was directed to here

I am trying to add an exchange fee of 4% to all currencies apart from Euros. However my base currency is COP (hidden and not in checkout) The point being that major currencies are worth much more and the exchange rates are very small numbers.

Taking AUD for example. 1 COP = 0.00040 AUD so to add 4% to that I need to add 0.000016 to the exchange fee field. I get an alert “Please enter a valid value. The two nearest values are 0 and 0.0001”

0,0001 is 25% of 0.00040 so that is equivalent to adding a 25% fee. How can I add a lower fee. This obviously wouldn’t be an issue were the base currency Euros but being able to store prices in COP is a huge advantage.

Ideally I would be able to apply a % value for the fee. The numerical value will invariably stop being 4% given fluctuations. Given that is not possible, how can I add a tiny number that is outside the permissible range?

Thanks