After hitting the PayPal button, you will be sent to the following page.  Here you can change the quantity.  Hit the blue Continue button.
If you choose to pay by PayPal, you will then be sent to the page shown below.  You can change or enter the Ship to address.  To see a breakdown of the charge, select the down arrow at the top right.  When you're done, hit the Pay Now button.
You will then be sent to the page shown below. Here you can log into your PayPal account or choose to pay by credit card.