I've added the modifier and saved the changes, but the box in which customers can write anything is still not appearing above the "place order" button. This can be done for a single item, correct?
Yep, it sure can @tgbauthor!
I've included some screenshots below of what this will look like when saved, just to confirm that you are looking for the text box in the correct spots. When a customer clicks on an item with a text box modifier, they will see a pop-up that looks like this (red asterisk if you choose to make the text box a required field) :

The text box modifier will appear like this in the cart once your customer has entered text into the field:

Is this where you aren't seeing the text box modifier, or are you looking somewhere else? Thanks!
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