The short answer to that question is, you cannot as we do not issue those or use them.
As usual we will offer you the straight and honest answer as to why which is that sadly the profit margins on spare parts do not allow us to offer discount coupons or codes and, even if we could for most items it would be pennies at best.
What we do is offer the best price that we can at the time we publish a listing or, if we see any price reductions as we pass these on to customers.
If you think an item is too expensive then please, by all means please ask us to have a look at it and we will happily do so or you.