How do you calculate a fee percentage to handle a fee being charged? Problem is that we get charged a 3% fee. We add this 3% fee to the invoice. When we get the amount back they charge 3% on the invoice plus on the fee we added. What formula can I use to figure out what percentage to charge on this so that we cover their 3% on both pieces? Example 10.00 charge. We add 0.30 to it to cover the fee, we send in 10.30. They charge 3% on 10 and on the 0.30 so they send us back 9.99 when we expected 10.00. How do we cover this? Also, how do we cover these discrepancies when they charge fees and do rounding per line item? -Edit- For clarity we are sending an invoice in where we know they will charge 3% on it, we need to add a fee to our invoice that will negate this 3%. It has to take into account that they will also charge 3% on the amount we add to negate the 3%. Need to know what % to add to our invoice or what formula to calculate this. Basically some sort of break even formula.

If $r$ is the fee (here $r=.03$) and $C$ is the charge you want to recover at the end, the amount to submit is $C/(1-r)$. So the mark-up rate is $r/(1-r)$, e.g., $.03/.97\approx.0309278$, or approximately $3.1\%$.

If you don't want to switch back and forth between percents and decimals, and prefer to think of the variable here as $r=3$, the pertinent formula for the mark-up rate is $100r/(100-r)$.