“Service” Items are best for non-physical sales eg labour/time (hours), professional fees or in your case, Advertising Space. Whilst you can still use quantities, these are not tracked - with a running balance - in the system. You can still utilise a specific service Item that you may buy in & invoice out as above.
“Other Charges” are more one-off or miscellaneous things eg delivery charges, late fees etc
NB: “Inventory” & “Service” Item types CANNOT be changed to a different type at a later stage!