How to allow ordering different sizes of the same product within one form submission

I have customers who need to order t-shirts for different sizes of the same item but it only lets them do one at a time so they are having to go back and fill out a second order form for the next size and having to pay me twice instead of being able to select the size, add to cart, then keep going to have it all in one order. Is there a way to add this feature to my form?

It sounds like you are trying to set up sub products, based on the T-shirt size that is selected. Yes, we can do this easily, by following this guide:

http://www.jotform.com/help/264-Create-Sub-Products-Based-on-a-Product-Option

Simply open the payment tool on your form by clicking on its wand-shaped icon, and use Next, Next button until you reach the product listing.

There, (1) click on the product, (2) click on pencil-shaped icon to edit the product option such as T-shirt size, (3) select the option to create sub products, and then (4) (5) finish the setup and save the change to this product.

Please take a look at the following image for visual guidance:

Your form would end up looking like this:

Please let us know if you need further assistance, we will be happy to help.