Yes! On GiveGab, you can charge (or offer for free) a large variety of items during the checkout process as a supporter registers for your fundraising event.
We call these items "Swag Items", and it is part of our Fundraising Event offering. You have flexible control over how the item is listed including the title, an image, a price (or free!), and setting a max quantity so you can limit how many each registrant orders.
Adding a t-shirt is a little bit different as it gives you the flexibility to add many different size options. You can learn more about adding t-shirts to your event here.
Here is a sample of items we've seen sold as Swag Items:
Upgraded T-Shirt Add-On