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.
On the widget, the checkbox field looks like this:
And the number field looks like this:
See these fields in action: