I hope that it turns out to be authentic, and that the shape is a result of use.
I know that $50 is not terrible considering how come people are tricked into buying fakes at over $1000, but if the bag IS fake and arrives by mail, you have to either keep it or return it (which is a real hassle). And it's $50 that could've gone to another purse
If the bag is fake, then refuse the parcel and file a chargeback through your credit card claiming that the item was identified as fake, and therefore refused. This way, you don't have to pay to ship the item back. Your credit card should protect you all the way.
If the seller is nice, then you may be abl to get a full refund and have the transaction cancelled.
This is, of course, if the bag is fake.