If you are seeing your UPC codes in scientific notation (something like 1.55E+08) after formatting the UPC column correctly and saving the file, don't worry! This is normal. What's happening here is Excel defaults to its usual (bad) behavior of formatting UPCs and other long numbers as scientific notation.
As long as you have formatted it correctly and saved it as a CSV, you've done everything right, even though it looks off when you open it back up again. All you have to do now is run that CSV through the UPC tool and get your results!