I don’t have a copy of the Ella theme, so I’m unable to give you specific instructions. That said, if you’re willing to do some digging yourself, I’m certain you could figure it out. Here’s where I’d start:
Duplicate the published theme.
Edit the newly copied theme and start digging through the product.liquid and collection.liquid templates until you find the elements displaying product price, ex {{ product.price }}
In this hypothetical, you would add your text right behind the liquid like this: {{ product.price }} + IVA