on the off chance that anyone runs into the same problem I had, I just figured out that a piece of code in my includes\templates\my_theme\common\html_header.php was loading a javascript function that was apparently interfering with One_Page_Checkout's shipping price updater function.

Code:
<script type="text/javascript" src="Scripts/animatedcollapse.js"></script>
<script type="text/javascript">
animatedcollapse.addDiv('support', 'fade=0,speed=400,group=pets,hide=1')
animatedcollapse.init()
</script>
it was this animated collapse that I only really need loaded for the home page anyway, so removing it from the html_header.php allowed one_page_checkout to function perfectly fine.

That took way too long to figure out.