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put the serial no in a different cell?
or use VLOOKUP() in an adjacent cell to find it from a list?
or add it to the end of the product name?

not sure why you want to add it as a cell comment anyway; if you don't want it to be normally visible then you could use alt-enter to force it onto the next line within the cell and drag the cell height to a single line (and align text to top of cell!)