You'll never be charged twice.
If you chose to checkout using the yellow 'Checkout with PayPal' button you will be taken to PayPal first to 'authorise' your final amount. This is because no shipping costs will have been calculated yet.
You are returned to WithoutaKey to choose your shipping option then to PayPal for finalisation.
You can miss out on this 'extra' step by either choosing a shipping option from the 'Estimate Shipping' box in the cart first, or just checking out normally via the 'Proceed to Checkout' button in red.