I always pick up a festival lineup t-shirt and a poster, plus whatever band shirts look cool. The prices are about what you'd expect at a typical show. Usually around $25 for a shirt and $40-50 for a sweatshirt. Headliner merch might be a bit more expensive.
If anyone is looking for a lineup shirt, the Sasquatch website has every color in every size currently, in case you missed out at the festival. I bought a Sasquatch shirt about 4pm on Friday and I know they sold out not even halfway through the weekend. I wanted a shirt for Alabama Shakes and The Head and the Heart but both had sold out in medium before I got to each booth. Luckily I was able to pick those shirts up at the bands' websites afterward.
I'm not sure about that, I was just passing on what they posted on the facebook. Someone posted "Where do I enter the code?" then 2 minutes later "Nevermind..." so maybe it's after the checkout page at the point where you pay?