The digital download seems to have a catalog error that causes sales tax to be charged, at least for Washington state. So watch out for that - it lops off a lot of the discount. The physical card is tax free.
Normally, gift cards are exempt from sales taxes and you pay sales tax on your purchases at the time the card is redeemed.
Interesting to see how much discounts people get in the US and other parts of the world.
Australia often have discounts of 15% (such as buy 2x$30 for $50), but you can get it as good as 25% (buy 2x$20 for $30). The best ones to watch out for are the "buy 2 and get the 3rd free" - which equates to 33% discount.
Then again, the app store often charges more for apps than US counterparts until recently (with the AUD$ above parity to the USD$) and songs and ebooks about 2x more expensive for no apparent reason...