In order to lock downloads, links, and embed codes until your client has paid, you first need to create or connect an invoice.
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This works automatically upon payment using Quickbooks, Stripe, or Square invoices, but you also have the option to create manual invoices that can be manually marked as paid by you or your client.
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Once an invoice is created or connected, you will have the option to "Require payment before download" in the project delivery settings. Reference the screenshot above for the location of this option.