If you need to download an invoice from your subscription, you can do so from your main dashboard.
Log in to the account with the admin credentials (root account owners are the only ones with access to the billing portion of an account), click the drop down menu under the user's name on the top right corner of the screen, click on Billing as shown below:
Then click on Invoices, select the invoice that you need, click on "Print Invoice", and select Save as PDF as shown below:
If you need further assistance, please email your request to billing@saucelabs.com along with your username and some details of the issues that you're having.