To answer this question simply – it depends on what you paid for them. If they were cheapies they will more than likely have plastic lenses and you should just get another pair, even if they are polarised as the polarisation will be compromised.
However, if you have a more expensive pair, such as a pair from the Oakley sunglass range, instead of getting scratches out of sunglasses you can order a new set of lenses from your supplier or from Oakley direct.
There are some who suggest that you can mend high quality lenses but if they are still under warranty, once you try this you will void the warranty. If they are out of warranty, there the suggestions mentioned is to make light circular movements using a soft cloth and white non-abrasive toothpaste. Personally, I’d just replace them as I don’t think toothpaste can get scratches out of coloured lenses without it showing!
You brought your sunglasses in the first place to protect your eyes and look good – so unless anything has changed either change your glasses or get new lenses. To try and get scratches out of sunglasses is IMHO a waste of time.