To do this, use fields of type Ckeckbox or Number.
The Checkbox in one booking allows you to select one item.
A field Number will select several units of goods.
Open the resource configuration.
Create a Checkbox or Number field.
They are configured identically. Enter a name.
Insert the Paid checkbox and specify the cost per piece.
If necessary, upload the image.
On the widget, the checkbox field looks like this:
And the number field looks like this:
Look in action and choose the right option for yourself.