@judycanby:
It sounds like your clients want to be able to “dump” the item information in from their own spreadsheet into your order form using one big copy-paste action instead of each cell individually. Is that right?
Disclaimer: I am not a pro with this. However, if that is your clients’ goal, to just dump the info into the order form, you could just use an Excel spreadsheet instead of a PDF form. You send them the spreadsheet and when they add the 20 items to the signage, they can paste the item info at least by column all at once into the spreadsheet, then you data merge it into InDesign upon receipt.
I have a situation that sounds similar to yours. I use InDesign to create real estate property brochures for my photography clients. So when a client orders a photo shoot with me, it would be great to have their customer information pre-filled (if they are an existing customer) and all they have to do is fill in the property info, at which point I could merge it to the brochure file. My plan is to build a basic MySQL database where clients can log in and place an order, with their info pre-filled from a Customers table, then the property info. When they submit the form, the database will email me an XML file that is ready to import to InDesign. Perhaps a bit complex for your purposes, but it sounds like it might save you some serious time on the back end.
Edit: I forgot to mention, David made a great post about data merging that can be found here:
https://creativepro.com/creating-a-contact-sheet-or-yearbook-page-in-indesign.php
That’s how I imagine your 20-item layout appearing on your customers’ signage, so hopefully it provides some help.