I would first ask how reasonable your combined fees really were. I once bought 6 items (each with a $2 shipping fee), assuming one fee would cover them all.
These were items that weigh less than a quarter ounce and are flat, and could be bent without causing any harm.
He sent an invoice for $12 shipping.
(He sent them all in a 41 cent envelope too).
I didn’t change his invoice.
I just sent him $3 shipping–total.

He sent the item and never said a peep about my sum.

So, you can stick to your original price, or make a counter offer (that’s too low, how about $X?).
You are within your rights, but doing it with tact might be a best plan.

Tags: shipping