This article explains how you can pay your invoice if you don't have an account or you don't want to sign in to pay it. You can do this by paying the invoice online with a credit card.
If you want to see all your invoices and manage your account you'll need to sign in. See the article How do I view my unpaid invoices?
You can pay your invoice without having to sign in a couple ways. You can click the 'Pay Invoice Online' button from the email notice you received or if you only have the PDF copy of the invoice simply click the 'Pay Invoice Online' below the amount due.
Pay from the email notice
Pay from the invoice PDF
After clicking on the Pay Invoice Online button from email or the link in the invoice PDF a summary page will open in your browser and you can click the Pay Invoice With Credit Card $ button to complete your payment. A payment receipt will be emailed to the email you provide on the credit card form and any Users on the account that have the Store Purchaser role.
Credit card form