I'd bet what kind of panties you'd receive would depend on when you send in the letter of encouragement. They're probably doing some sort of weekly rotation, one week it's color 1 and design A, the next week color 2 and design B. For every letter you send, depending on the quality, you receive a certain amount of 'panty-points' which are what determines how used the panties you receive are, what way they've been used in, and by whom. An example would be that if you're a rookie, you get a pair of panties worn for maybe a minute or so by a fat yaoi fangirl. If you've accumulated insane amounts of panty-points there might even be a chance that you get to somewhat choose these kinds of things. For example, a pair of panties that have been worn when a loli got her first period.