Opencart by default is setup to allow multicurrency purchases, after you have installed other currencies and set their rates in relation to your local (default) currency.
This is good if your customers are from around the world. However if you only want their purchases through your store to be charged in your local currency, then I’d recommend a slight code alteration – note this tip is for Opencart 1.5.x + carts.
Line 26 or thereabouts:
Add after this:
Replace the ‘XXX’ with the 3-letter abbreviation of your local currency.
This will cause the customer’s order totals to default to your local currency on the final confirmation page and be charged using this currency at your payment gateway / processor.