Well, looks like I am going to be the odd one out again, which is usual around here.. LOL

MDF is a craft wood. You really cant describe it as being a wooden sign though! Technically, it is pulped up pine wood that is mixed with waxes and resins (or something like that) to make a medium density fibreboard  aka MDF 🙂

Lets face it, even cardboard was wood once, but you wouldnt go to the lengths of an arguement about that would you?
I feel that you *misled* your buyer with the description, and she has every right to be peeved about it.

And I dont think a *store credit* for a buyer who has already been stung by a misled description of an item purchase is going to smooth things over, even though you offered her refund and not the store credit.
For sake of face, I would have said keep the sign and here is a full refund too, so sorry you were'nt happy.

I also think you should make every effort to correct your items descriptions. despite a few people on these boards, a lot of people have heard of craft wood, which is MDF, so you can just say they are made from  6mm Craft wood (or what ever mill size you use) 🙂

cheers... 🙂