Fee in the gift, or on top

Same choice as a ticket, per event. Include the fee in the gift, the default: a guest who gives $10 pays $10 and the fee comes out of it. One clean number, which is what a tip jar should look like.

Switch it off and the fee sits on top: a guest who gives $10 pays $10 plus the fee, and you keep the whole $10. The card shows the total they will pay, "with the fee", under the amount, and the receipt lists the fee as a service fee.

Either way the fee is once per gift, and the floor you set is on what the guest types, not on what they pay.