We also want to sell our T-shirts in the reservation form. Is it supported?

To do this, use fields of type Ckeckbox or Number. The Checkbox allows you to select only one item within a single reservation. And the Number field will allow you to select several units of the product.

Open the resource configuration. Create a Checkbox or Number field. They are both configured the same way. Enter a name. Set the Paid checkbox and specify the cost per unit. If necessary, upload an image. Save.

On the widget, the checkbox field looks like this:

And the number field looks like this:

See these fields in action: