How Do I Create A Pre-order Gallery?

I see that some users have created galleries where clients can pre-order collections and prints, and then the client can go in later to fill in what photos they want for the items they pre-paid for. I'm just not finding how to do that.
I can see that when I create a collection, I can let the client check out without having to select photos for the collection, but that's all I could find pertaining to a pre-order situation. Is there an article or video I missed that goes over the steps on how to create a pre-order gallery?

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Here is an article on that:
https://www.picturespro.com/sytist-manual/articles/setting-up-a-section-for-pre-sell-packages/

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That's perfect - thanks! Is there a way to pre-sell a la carte items like prints? Do I need to create collections with just the one item in them?

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You will need to create collections for the one items.

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