Moderator fee should included an (optional) minimum fee in addition to a percentage

[lunokhod]

Brief Description: In some cases, it will not be worth the time of a moderator to investigate disputes for items that have low prices: If an item costs 1$ and the moderator has a 1% fee, they will get 1 cent. The risk is that the moderator will not respond to disputes that fall below a certain amount.

Solution: To resolve this, the moderator should be able to set a minimum moderator fee (in absolute units) as well as a percentage (which would be used whenever the fee is above the minimum value). As a result of this change, the moderator should not be allowed to be chosen if the moderator fee is higher than the price of the item.