Can I open the supplier's CSV file instead of copying from it?

Yes. In Paste a list, press Open a file… under the box (or drop the file on the dialog) and choose the .csv, .tsv or .txt file, up to 5 MB and 5,000 lines. It is read in your browser and not uploaded. Commas, semicolons (as Excel saves a CSV in much of Europe, with prices like 12,50), tabs and vertical bars are recognised, and the dialog says which it used. Accents survive a CSV UTF-8 file and an older Excel's plain CSV saved on Windows in Western Europe. The rows fill the box, so you choose SKU is in and New value is in as for a paste, then Check the list. An .xlsx, .xls, .ods or .numbers file has to be saved as CSV first, and a file with more than 5,000 lines has to be split.