Are you running a buy one, get one free promotion? There are two ways you can set up a BOGO deal with a promo code.
1) You can create a true BOGO discount that will require the customer have at least 2 tickets in cart, and will provide a 100% discount on only 1 ticket. Here are the settings to use for a true BOGO discount:
Type: Percent
Amount: 100
Max Uses per Order: 1
Minimum Quantity: 2
2) If you want to allow the discount on more than 1 ticket per order, you can use the following settings instead. This will require the customer have at least 2 tickets in cart, but will not be a true BOGO as it can be used with odd ticket quantities:
Type: Percent
Amount: 50
Minimum Quantity: 2
