How can I automatically check a product item based on an answer from a checkbox field?

I would like to set a check box to yes when a client chooses yes in a different box on the form. I have conditional statements for other boxes but it seems I cannot check a box for the group of payment choices.

In the attached URl there is a form which has a T-shirt check box. If they box Yes is check I want the box yes check for the T-shirt payment option .

Unfortunately, it's not possible. You will have to advise your client to choose the item from the payment field. I tried using another checkbox field for the product item, but you have a property in the t-shirt item so this method still gonna won't work.