Once the invoice is generated click the Items button (as seen in the image below)
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You will then see the items in a dropdown with a checkbox for collected
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In the example image below the admin user could tick one or both items as collected
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You can also click the Mark Collected button, found in the Actions column, to mark all items as collected in one go

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The items are now marked as collected (see the green tick icon below)

If you had only marked one out of the two items in the top example image, the number of items collected is displayed in the Collected column (see image below)