source?
In halacha the sake on Amazon is not a kinyan, neither is his accepting the money. Once he ships it knowing the sale price and the buyer makes a kinyan it is his. Since he knew the mistake before shipping it is not taos. Nothing gives the seller the right to the extra 300. The buyer was admittedly taking advantage of the situation, but the seller outright stole the 300.
Not the Torah view regarding:
I understand that the business is Jewish, but they are in the end just that, a Business.No, it is a
Jewish business.
If they don't keep up to their word then you do what you need to do.No, you do what Torah tells you to do.
Im sorry that I might hurt a fellow Jew, but ...No buts, you don't justify hurting another Yid unless the Torah tells you that it is justified.
In cases like this, you need to speak to a Rov.
Regarding what you write about a kinyan; do you have Smicha Yodin Yodin on Choshen Mishpat with Shimush? If not, the question should be asked of a Rov who does.
There are other Halachic issues here in addition to those you mention.
Also, you don't know that he knew about the mistake before shipping. If he knew, he might not have shipped and returned the money.
A Rov may argue that he isn't 'stealing' $300. He is taking $450 for something that is worth $450 and which both sides agree that the 3 for $450 was a mistake. The buyer thought he would get away with the mistake. He didn't. The seller caught on that he made a mistake. He can always return the item and get a refund. Maybe he shouldn't have opened it until it was resolved.