I just renewed my Borders homeschool discount the other day - were you behind me? LOL
Anyways, in OR you don't have to have anything official filed for homeschooling until age 7, so I don't have anything documented. I just tell them that and I have to fill out a little form and get my Border's 20% discount card. With Borders it really depends on the cashier at how good your discount will work. The last guy put all my parenting books through on the discount too which was nice. Some folks make you pile them up seperately and run them through one homeschool lot and then pay the rest another transaction - irks me. The worst time was I was buying stuff with our homeschool discount for the kids, stuff on my executive discount for DHs business, and magazines. The old broad made me do 3 transactions (the executive discount is the same % as homeschool so i wish she'd just run it all through as just one - which others have done).
We signed up for Joanns online, but I've never used it yet. It came with a coupon too in the mail.
Our local grass roots bookstore does a homeschool discount too. I wish the toy store did though - the guy that owns it homeschools! and gets lots of great educational stuff there (math manipulatives, books, etc), but no discount.

Maybe I should leave a note in a suggestion box. LOL