I do custom orders. But the way I do it does not sound like what you are thinking. I have customers submit their information through a contact me form. I email back an invoice estimate. For tracking purposes I have created a Custom Order Category, and have created custom order items in $5 increments that are hidden so they do not show up on website, but I can still add to invoices and send a payment link to customer for payment.
But it sounds like you need to just turn off inventory tracking and that would allow customers to buy online and it then gets printed, correct? You could then create a new category called trade show or something like that in the items section of square, not the online items section. Categories in the items section and online section are different. A category created in the items section will not show up online as a category online, only on square app and for reporting. Then you could make duplicate items of the items you are going to carry stock of in the item library, but make the duplicate hidden instead of visible so it doesn't show up online, only on the square app for when you sell in person. Then you can activate stock tracking on those duplicate items you will be carrying stock of and tracking. Then finally, you add those duplicate items to the trade show category, or whatever name you want to call it. Hopefully this helps. This is my website.https://mandysclaycreations.square.site/ yep shameless plug I know lol
