It's much better to be frank about your disappointment, but suggest feasible alternatives. It's pretty clear that you are not going to get your jacket for $180. Maybe your fallback position is to get a coupon on something in season, in addition to the jacket for $300. The margins for in season goods are much higher, and you are more likely to get leeway. And never demand it, even if you feel entitled to it. Phrase it as asking for a favor. Here is an example:
"I really wanted those boots, and $180 sounded like a great deal. I guess that it was too good to be true. Unfortunately, $300, even though a very good price, is outside the pricerange for me right now. Had I known that there was an error in the pricing beforehand, I would have bought something else, such as the SNS Herning sweaters that were on sale for $120. Do you have any of those left? Would it be possible to get a coupon (maybe 25%) towards a future order? Thanks for understanding my situation. Yours, noob."
That sort of conciliatory attitude will get you a lot further. Attacking people invariably gets them on the defensive, and you'll probably not get anywhere.
amen...people that attempt to get what they want via force/strength baffle me.
ASK them to give you what you want, even better if you give them an option or two--you'd be surprised how much better the result will be when asking versus demanding, using an otherwise identically worded missive.