I don't know about discount codes but can recommend Adamesh. Bought a pair of stainless back boxes for my S Type last year, took about 3 months before i found the time to fit them only to find i'd been mistakenly sent a pair of S Type R boxes by mistake.
They sent the correct boxes through and then collected the wrong ones a few days later once i'd fitted the right ones and packed the wrong ones into the box. Very pleasant to deal with, no problems at all on doing the swap, excellent all round.
If you're paying via Paypal, you could sign up for honey.com, it finds and applies discount codes to many online purchases so you may be lucky........