Hazmat Exclusion on Free Shipping with Dynamic Codes

Hello everyone! 

I started a rewards program for my company in May and we are still working out some of the finer details. 

One of the perks we offer is one day of free standard shipping every month. Currently, I do this by sending a unique code to everyone in a launch email. It has been working, but the one stipulation is that we exclude Hazmat orders because we lose money. 

We are currently stumped on how to prevent people from ordering hazmat items while preserving the dynamic code. If anyone has any insight or tips or (hopefully) solutions, I am all ears.

Thank you in advance! 

Jenna