Indeed this is a case of greed on the states behalf. I live in new york so I suffer the same fate. Something about that screams illegal being taxed on the same thing twice. This is an online code for currency, not content. I don't think that can be taxed unless we found a way to tax currency exchanges. You wanna step up and offer a solution or an answer amazon?
I just bought a code and I was not charged sales tax by Amazon. Also, I haven't been charged sales tax in the PS Store for a while. I currently live in New Mexico and used to live in Ohio. Don't know if there might be different policies for different states.