I can confirm what Stefan says: we have services configured with a UoM of "each", and with an Item Group type of "SEFL" (Service - fixed price without actuals).
These can be included on a regular sales order and do not require any type of availability confirmation.
You do need to (manually) "Confirm Service Execution" (under the Sales Order "Submit" button) in order for these line items to be invoicable.
Best regards,
Eelco