they didnt advertise them as a set, you ASSUMED they were sold as a set. That it not false advertising.

As Holly pointed out, when they are offering them together, they price accordingly on the signage (2 for $X) but in this case, it said $19.99.

The fact that you opted to pay and walk out of the store with the merchandise indicates that you either a) realized that you presumed incorrectly and didnt want to embarass yourself further, or b) It wasnt a big enough issue to speak up about it and notify the manager. If it's me and I truly believe the pricing is wrong, I'm going to speak up right then and there! My money is too hard earned to blow on a misunderstanding.

But to write a letter like this after the fact demanding "restitution" is a little suspect.