When you automatically use PayPal account to pay for shipping labels, you won't need to log in to PayPal, and the shipping label cost will be charged to your PayPal account.
To automatically use your PayPal account to pay for shipping labels, you need to have a credit / bank card on PayPal as a backup funding method. Your PayPal balance will be used for the label cost. If your PayPal balance doesn't cover the label cost, then the backup funding method will be charged.
If you want to stop this automatic payment option later, you can log into your PayPal account to edit your preferences.