Used Attitude several times, with the T-shirt stealth method and the tin stealth method. With at least one of the T-shirts, I had them keep the seeds in breeders packs which vary in size and style by the breeder. With the tin method, I had them take them out of breeder pack and anything that wouldn't fit in the tin was in a small plastic bag (3" x 1.5")with a label sticker so there would be no mistaking which seeds are which. By experience, the breeder packaging is really unnecessary but likewise probably doesn't hurt anything either, it can just be larger than you expect...I don't need a whole 4x4 piece of cardboard to hold 10 seeds haha. Greenhouse is on the big side, Mr. Nice was small foil packets, can't remember any other ones.
My guess is that Pic n Mix will be treated like their UFOs, which will be the small plastic baggie and maybe the bags stapled to a piece of cardstock.
Good luck!