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:
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