Thank you for posting your fix! Worked for me too. I was having the same problem getting a 1054 - Unknown column. The Discount Code 2.6 for osCommerce 2.3.1 (full install pkg) is missing this "customers_id" line in the SQL Query step 1.
So if you haven't installed add `customers_id` text, right after: `excluded_products_id` text, before running the query. Or just go to your myphpadmin and add a row in the table.
I must be missing something because I tried this and it still does not carry over..
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[add-on] Discount Code (support)
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I must be missing something because I tried this and it still does not carry over..