Taxes and Fees option
From the setting section, you can easily enable the taxes and fees option in modern event calendar plugin. Over there you need to create the fee with your own specific criteria. And the tax setting is identical to the fees, you should just alter the title. Plus, you can activate coupons from this part.
To use this option, go to MEC Settings > Booking > Taxes/Fee . You should enable this module and add a fee to it. The fees will be fixed if enabled in the single event.