How do you set-up a coupon for money off the total order?

Based on research that shows sending patrons a promotional coupon worth a flat dollar amount does better than percentage off deals because recipients think of them as leaving cash on the table, we're implementing a Ballet Bucks campaign for our Nutcracker single ticket buyers. The promotion will be worth $50 off their next order with a hard deadline. I'm struggling with how to implement this though. Here's my thought process:

• Pricing rules and discounts apply per ticket and not the total order, so they won't work.
• Allocate works within the application, but not online. It also applies the $50 towards Contributions and Memberships, which we don't want to have happen. Additionally, it would require our Patron Engagement (Box Office) staff to subtract the $50 from the order total before clicking Allocate and using that amount for the Allocate amount. I want this to be easy and error proof.
• It doesn't look like negative dollar amount fees are possible so that's out too.

The promotional piece is ready to mail, so I can't change the offer. I'm hoping you've done something clever that I can shamelessly copy. If it works online in TNEW too I'll owe you one. Thanks!

*Also posting in the Marketing forum.