I've made this correction on the plugin's GitHub repository and updated its current version to 1.0.4-beta2.
If a customer "hits" your site using a version of IE less than 9.0, they'll continue the checkout using the "traditional" multi-page checkout due to incompatibilities provided by those older browsers' javascript support.
That change is "booked" for the plugin's v1.0.4 release, coming to the Zen Cart Plugins RSN (real-soon-now)!
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