To have the invoice click on your store name on the top right corner of the Stock Sync page and click Manage Billings.
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.