Surcharge as Percentage with a Minimum

Would love to be able to create a surcharge that is a percentage of the rent with an absolute minimum charge. For example on the LDW there would be a minimum charge of $27.00 but I would like to be able to charge XX% of rent. So for example I would like to charge the fee as 3% of rent. On a 1000.00 rent charge it would be a charge of $30.00. If the rent were only 500.00 then the charge would be $27.00 instead of the calculated 3% of $15.00.

As an added thought to this it might be nice to have not only the minimum but a maximum as well.