Either you pay for the item/shipping directly or you top-up your account.
On the payment page you choose your payment method, if you topped up you can pay from your balance.
For some restricted items, Paypal won't be available, but you can still with Paypal by topping-up your account and then purchasing the item with account balance.
You can always close this page (for instance to top-up) and come back by going to your shopping orders.
This will redirect you to the payment gate of the service you chose.
After paying you should check your orders page to make sure everything worked.