Shopify apps cannot run in the checkout and the checkout currency is always your store's default currency.

This is why, using our app, you can add 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.