Hi,
This can be achieve like below:
When you use FERT item in sales order, item category is TAN (or any customized as you use in your sales order) which is relevant for pricing as well as billing too. The schedule line which is aligned with this item category is relevant for availability check and stock should exist in you plant.
When you use service item (DIEN), the relevant item category is TAD (or any customized as you use in your sales order) which is also relevant for pricing as well as order related billing. Here you can change the item category in your sales order manually as TAW (by determination of item category in VOV4) and that will get copy in to delivery (no stock require for this), PGI accounting will post for FERT only and both the items will get copy in to billing and accounting.
Regards,