Well THAT'S convenient! PSN no longer charges me tax!
$5 is the minimum you can put on the store with a credit card. As for the tax I thought they stopped charging it back in 2011? Well for me anyway.
I was told it is determined by state and stuff.
It used to be. However http://www.ps3trophies.org/forum/general-ps3-discussion/114612-psn-store-no-longer-charging-tax-3.html
He/she explains why there isn't a tax any more pretty good. Basically unless the servers that you download the games from are in your state then your state can't charge a sales tax even if they have a sales tax for digital goods in your state. At least that was my understanding of it. I just know I used to hate the tax because if I got a $20 Sony card I couldn't buy a 19.99 game. Then it just stopped charging tax.