I've used a contents-by-kit list after an earlier finger-count dispute, so our missing additional FR5 tool cable is clearly recorded this time; the seller acknowledges it is owed but keeps saying next shipment, and I need an actual supply option instead of another acknowledgement
Our supplier once used next shipment to mean the next order we placed. Ask whether this is waiting for stock, waiting for a dispatch booking, or waiting for you to buy something else. Then request standalone supply against the existing shortage.
I would ask why any additional charge is needed for supplying an included item before accepting it. The immediate goal is a definite answer, not volunteering to pay twice just to end the emails.