The link to purchase the app will only show when user have reached the limitation of the Free Edition. (Up to 3 invoices).
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(1) Create at least 3 invoices or estimates.
(2) Then try to create an invoice to trigger the limitation check.
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(3) This will direct you to the purchasing page.
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(4) If you decide not purchase the app and continue to use it, there will be a warning banner that will be displayed that is linked to the purchase.