Incorrect amount from credit card taken when sold in dollars

We have a test product that sells for £1 plus shipping of £1.50 = £2.50

When a user changes the site to show prices in dollars, the price is converted to $1.56

But when purchased by Credit card via Paypal the received amount (and the amount paid) is $2.50 and not $3.90

The amount is shown correctly in Opencart as $3.90

Version 1.4.9.1

Any help appreciated.