CW promised $10 off on merchandise purchased by a certain date. They did not promise $10 off of a specific item. The OP could buy anything else and receive the discount. I believe CW is not required to honor the coupon past the expiration date.
It is unfortunate CW is out of stock on the shower head; however, the sale catalog is made up far in advance of it reaching our mailbox and items expected to be on sale are not available. This is not unusual in the retail, catalog business. CW could give a raincheck on the sale item but I still don't believe they're required to extend the coupon date.
I agree there is something wrong with on-line purchasing. The site should clearly indicate out-of-stock items before you put in your credit card information.