Just as a cosmetic quick cure.... it will look a lot better if you give the whole pannier a wipe with "Duck Oil" the wonder product for cleaning anything (except brake parts) but it works really well on black plastic. Available in spray or I buy 5 litre at a time, it is much cheaper in bulk.
This will not take the scratches out but hide them somewhat. Then just forget about them till next time!
good luck, regards,
Barrie