Consider the upside of giving them away - (As a loss leader)
For good or bad I think I break most of the "rules" of running a freelance business daily. That said, I've been running a successful business for fifteen years, so I must be doing something right!
In this case - based on the assumption that someone printing a t-shirt design would likely want to design other t-shirts, and likewise that someone who has a use for three logos would likely have a use for five...ten...a hundred... I would be inclined to offer the other artwork for free (providing it takes no more time to finish). Or else a very low fee based on the time it takes to move them from comps to finished artwork.
Building goodwill, especially at no actual expense of your own (you already have this artwork after all) is a massive bonus in our business, and part of the reason I have never had to advertise my services in all these years.
Making yourself, and by proxy your business, fair and likeable is more than half the battle.
_ Quick edit to say that obviously there are many other factors that need to be taken into consideration here, including time you've been with the client, their general attitude, the time the work took, the profits they stand to make... so I cannot give exact advise, but I do think my suggestion has merit "in general".
_ Quick edit II - It's important here that people understand I do not advocate working for free (boo!) What I am pointing out is that a "loss leader" is a well founded strategy to build good-will amongst a client base generating greater long term profit (yay!)