Making first item free in order form, then charging fee for additional items

I am creating an order form for people to order t-shirts for a group event.  Each person is entitled to one free shirt, but will have to pay for additional shirts.  How do I create an order mechanism that allows them to choose the style and size of their choice for the first shirt and now be charged, then to set a price for additional shirts?

As I can understand, you only need to set the first ordered shirt as free, but then charge for additional shirts, you may set up your product on your form and then set up special pricing, so if the user wants more than one shirt then he will be charged. 

This guide will help you to set  up the products: https://www.jotform.com/help/165-Setting-Up-Your-First-Order-Form 

And this one will help you adding the special pricing: https://www.jotform.com/help/265-What-is-Special-Pricing 

I hope this helps.