Problem with calculating ship price for Pound/Euro

Hi. I'm using an OpenCart v1.5.1 for an online store (started running few days ago) and I came upon a problem. Here's the thing:
When someone is buying a product and goes to Checkout at "Step 4: Delivery method" they can only choose USPS, because that's the only method I have made available. And depending on the weight of the product and the delivery location it automatically gives a price -> $26.25, £10.82 or €14.69. But when the buyer reaches "Step 6: Confirm order" it shows the products, plus Priority Mail International: $26.25, £16.85 or €19.64 !

For whatever reason at Step 4 it gives lower delivery price when Pounds or Euro are used. Do you have any idea how I can fix that? What causes this to happen?