You may set up your event by making it so ticket purchasers cover the processing fees. 

To do this, begin by selecting Processing fee paid by the ticket purchaser in the column pertaining to Ticket Purchases.  If your event has enabled Donations, make the same selection.

Next, select the box that gives purchasers the option to pay the processing fee during the ticket purchase.  You will see that you may also add a bit of text to create a custom prompt.

When the purchaser arrives at checkout, your custom prompt will appear.  If they elect not to cover the processing fee, they simply uncheck the box before entering their payment information and completing the transaction. In this case, your organization will absorb the transaction's processing fees.

When supporters reach the checkout page, your custom prompt will appear as a preselected checkbox:

If they uncheck the box, they will not pay the processing fee; your nonprofit will absorb it.