Add each product with its name, price and quantity. Every product gets its own barcode, and the quantity is how many identical labels are printed. Each label shows: logo, barcode, product name, price.
The same name and price always gets the same barcode on this browser, so reprinting an item later keeps its barcode.
Random, unique EAN-13 barcodes with the logo on top.
In the print dialog set margins to None, scale 100%, headers/footers off. Thermal roll layout: paper size = your label size. A4 layout: paper size = A4. Test with one label first and scan it before printing a big batch.
Codes are built from a scrambled minute-by-minute time stamp plus secure random digits, so a code made in one minute never matches one made in another minute (for about 19 years). EAN-13 codes start with 2 (the range reserved for in-store use), so they won't clash with real product barcodes.