After a redundant and circuitous dialogue with the support team—one that I imagine many of you have similarly had—I was directed to reference the "Completing the product import .csv file" article, ensuring that all allegedly required columns were filled on my sheet. 'Product URL' and 'Categories' are, for example, among the so-called required elements despite the site's ability to auto-generate URLs for cells left blank and to hold shop items without category tags. The "Missing column, Add PRODUCT ID column" error message clearly takes issue with that particular leading column, making it especially confusing when the support reps directed me to the Product URL column and others after it—but ultimately the whole thing is a glitch.
After creating upload sheets in both Numbers and Excel, saving in multiple formats and adjusting in minute ways, I found that a .csv upload will work successfully if the file name remains unchanged from the original template filename given by Squarespace, "product_import-v2.csv". This solution worked for me three times in total after innumerable instances of the "Missing column, Add PRODUCT ID column" message, and though I certainly can't explain how it circumvents the glitch that lives in the system's upload process, I hope it well help some of you!