How to handle a service that I sell but do not buy

I offer consultations 1 initial and one follow up . Each one has a different charge.This needs to be invoiced. I would like to handle them like an inventory item rather than time billing because the price is not time related. Is there a way of handling this as an inventory item or some other way. I imagine this might need to be handled some other way since inventory is for physical items.

Yes, go to Settings, then enable Sales Invoice Items which is a module built for this purpose. Once you create your sales invoice items, you can select them on invoice form.