The Portal is where you can go to find your previous Invoices.
Invoices are on the subscription page
Once you've signed in, navigate to the Subscription page, scroll down and there they are:
Click the blue download link next to each invoice to get a PDF copy for your records.
Next invoice
When you change plans mid-cycle, your next invoice is likely to get a bit complicated due to the pro-rationing that happens automatically as part of this change process.
To get a preview of what this is going to look like, click the show details button on the blue bar below the Invoices heading: