I see on your site that you do offer a MLP pattern for sale, so really this situation depends on what you would have to design (it seems like the pattern would be at LEAST half the battle of plush-making) and how complex it is, whether they intend to reproduce it or if you're going to keep it to make plushes off it, yourself, and so on...

I think it's a judgment call on your part, but do be sure to write up a specific pattern-commission contract with clauses you feel comfortable with, and if it's a complex design, I would highly recommend you be sure to charge accordingly! Your stuff is worth it; SUPER cute!