I've searched high and low for answers to the problem I was having with no luck. I've solved the problem and I'm posting both problem and solve here for the people that come after me :)
My problem: I couldn't figure out where to add a currency prefix to the prices Instagram Shopping was displaying. My shopify shop currency is Mexican Pesos. But for some reason, instagram was showing my prices in US Dollars, the default currency. Instead of displaying MXN$800 for a t-shirt, Instagram displayed $800. Which naturally lead to the occasional upset customer posting comments like "$800 for a tshirt?!?!". The problem was that customers were assuming prices were in US Dollars. What I needed was the MXN prefix to denote Mexican Pesos (and not US Dollars).
So, to change the currency symbol and add a currency prefix that your products display in Instagram, you have to specify a country in your Catalog Manager, in your Facebook Business Manager. You do this as follows:
The Bad news: Unfortunately, Instagram hardwires the currency into each post, so for the old posts you've already tagged products to, the currency will not change, you have to go back through each post, one-by-one, and retag the products to get the new currency prefix, symbol and value.
I hope this helps.