Is it possible to link products to a billable item so that it automatically included in invoices and reduces the stock?
There isn’t a way to link a product with a billable item but you can link a product to an Appointment Type ! That way, any time you create an Invoice for that Appointment Type, it will now include both the Billable Item and Product (which will also reduce stock as you mentioned). To do this, head to Settings → Appointment Types then select the “Related Product” (under “Related Billable Item”).
As an extension of that, is there any possibility on being able to link multiple billable items (rather than 1 billable item, 1 product) to an appointment type?
This would be useful for say appointments which include report writing as a standard inclusion and being able to have a seperate line for report writing + consult times automatically included.