How to download the invoices for the subscription purchased?

First, sign in to the Admin Center using the same username and password you used when you bought your subscriptions. Then, click on your Profile name. A menu will pop up. Click on the word that says 'Billing'.

When you're in the Billing section, look for 'Billing History' and click on that.

You'll see a list of everything you've bought with your subscription. Find the 'Download' button and click it to save the bill as a pdf.