To have the invoice do click on your store name on the top right corner of the Stock Sync page and click 'Manage Billings'.
Then click on the "Manage Payment info" link:
Then a link from billing.stripe.com will pop up. You will need to scroll down until you see the "Billing History" as per the below screenshot:
Once clicked on the button, it will bring you to a page where you can download the invoice:
Please give it a try and do share your thoughts or ask us any questions if you may as we intend to make your life easier with our applications.