After some frustrations with conflicting documentation, I figured I'd make the blog post I wished I had at the start of this all. This forum post is what finally led me in the correct direction.
If you're using Universal Analytics through Google Tag Manager and are tracking Standard Ecommerce transactions in multiple currencies, you'll need some extra setup beyond the documentation they provide. You'll notice there's no mention of setting the currency when sending a standard ecommerce transaction because the Universal Analytics through Google Tag Manager doesn't support it. Thankfully, it's easy to add.