In Shopify, the checkout currency is always your store's default currency. No currency converter is allowed to modify the prices in checkout.
We are able to help by converting the total order value in local currency and showing this as well, this is pretty much everything that a currency converter can do in Shopify's checkout.
We also a notice to the cart, letting the customers know that they will be charged in your store's default currency, even if the prices in the site were converted in their local currency. Even more, every time our app converts a price in a local currency, the value in the store currency is visible when they move their mouse over the converted price.