You are fine. Solid food 20 minutes after you workout has been used for decades before the relatively recent notion of getting all immediate workout nutrition in the form of shakes. I would assume, especially with Plazma before working out, that you are fine.
Don't stress out about it. Free food is not something I would pass up postworkout just to down a shake costing a couple bucks. Your gains may be effected a grand total of 2% by eating solid food instead of shakes, and even that much is probably a stretch.
Plus, since you can't move the workout timing or the free buffet, your only other real option is to skip the free buffet for your own post-workout shake. If you really want to spend extra money on something that will not likely make a drastic change in your body composition, go ahead. Just seems crazy to me. You could also have a shake or something immediately post-workout then go eat 20 minutes later but I don't think that will really make a difference honestly.