My copy of my invoice and the email confirmation is at work so can't remember how I got it. Have you tried logging onto your MY Account and reprinting your order. I just did that w/o actually printing it and I saw the discount code.
BTW SNM, the discount is 20% I believe, however, your announcement at the top of all the forums says 10%.
"You will also receive a 10% off coupon that may be used for future orders (May 1 thru May 31st) on our ShopCathe website (STS items are not included with this offer)"