Yes, you can order replacement slipcovers for a fee. You can also order replacement cover art. I don't recall the phone number, but the info can be found at the Blu-ray forum Disney Movie Club thread here.
My Walmart also had Incredibles for $24.96 and Cars for $22.96, but to match Best Buy's deal they took $5 each off of $24.99 as they were listed in Best Buy's ad.
So I got Incredibles: $24.99 - $5 - $10 = $9.99, Cars: $24.99 - $5 - $8 = $11.99. Would have been nice to save the extra $2, but...
It's been a few years since I watched all my DCAU DVDs, but IIRC there are a few episodes of Batman Beyond that have some appearances from Justice League, and one episode of Justice League acts as an "end" to Batman Beyond. Pretty sure I watched in this order:
Batman: TAS
Superman: TAS
Batman...