In any Payment Form, such as a Ticketing form, under Payment Information, there is a selection called Payment Section Visibility. This allows you to have a form that can be submitted with or without a monetary value. For example, if you had a ticketing form that offered both Free and Paid tickets on the same form, this function will make that process easy.
By putting a check by "Hide the payment information section unless the Total Amount is greater than $0.00," if the total that is displayed at the bottom of the form is ever $0.00, then the payment section, asking for a credit card or PayPal will be removed, so your supporter can still enter their information and order free tickets. At any time when that total is more than $0.00, such as with the purchase of a paid ticket, adding a donation to the order, etc. Then the Payment information will appear so your supporter can then pay.