I'd probably offer the buyer a partial refund (since it was my mistake), or maybe a discount off future purchases from my store. Most people are really easy to work with if the need ever arises.
Honestly, this is why I never grade things in the titles (aside from New if it's still factory sealed). I actually almost never use Like New unless I had to remove it from the shrink wrap myself for some reason, and use Very Good sparingly since it applies to the jewel case as well (which is stupid and needs to be changed). I'll use the seller notes to describe the condition of the disc specifically. It seems to be working well for me.
I feel like taking really good photos and filling in enough information to be found easily are the two single most important things you can do. Unless CDs or DVDs have a lot of competition (in which case it probably has little value and would be better selling in a lot), I really don't think the grading makes much difference to buyers as long as you have something somewhere that lets them know the disc itself is in really good condition. Maybe I'm wrong.
