When in the Self Service Center (SSC), you can navigate to the Invoices menu, as seen in the image below. This shows a list of invoices for that subscription and their statuses.

Below are some more examples of different statuses:

Below is a picture of a credited invoice:
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Below is a picture of an open invoice:

Below is a picture of a pending invoice:

For more ways to handle your invoices, Firmhouse's guide on paying outstanding invoices via payment links explains how to use unique payment links directly from the invoice page.