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:
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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:
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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.