How to create a header level pricing condition

Greetings. We have a requirement to add a header level pricing condition and we're not sure if this is possible.

The description of the requirement is:

If customer uses promotion code XXX and the order is over $50, give them $5 off the shipping.

We've done plenty of line level conditions but never have had luck with the header level. I think the main issue is that we are not sure how to get the order total into the requirement at the header level.

Suggestions?

Thanks!