I have had available mean a warehouse enquiry rather than allocated stock. Ask whether your exact spare is physically available, and what separate dispatch would cost if it is.
Send the order line for the additional cable, with the installed one marked received. Before discussing freight, get an acknowledgement that one extra item remains owed. Otherwise you're negotiating the shipment of a misunderstanding.
Ravi, worth asking, but Jane needs the whole delivered replacement price beside any credit offer; matching the seller's item price could still leave a shortfall.
Has the seller answered with a revised amount or a real supply option? At least purchasing now knows the uncovered cost rather than treating the missing spare as free future stock.