With gst senario hav e changed for gst.
Now one can also sell service to customer.
In stores we sell services like membership service,tailoring service,drycleqning service to custmer.
And it should apper on bill as service and bill qty should not be added. If service is sold nor the inventory should not be there for service barcode.
I would suggest you to think for concept pf service invoice at pos store /site
You may create a non-inventory item and then sell the same. Bill quantity or Inventory will not be affected by the same.