My favorite Mexican food restaurant has "NO SUBSTITUTIONS" posted at the top of every page of the menu. Why? It makes things more difficult for the servers and the cooks. When you order the "Dinner #1 Beef" the server just has to write down "#1 Beef" and the cooks know exactly what goes with it (by the way, it is 2-enchiladas of your choice - beef, chicken, or cheese-, a chalupa, rice, refried beans. I can't' even tell them no tomatoes on my chalupa, I just pick them off). They have a HUGE menu and it is too tough to do subs. I see the same thing here. You want to sub the case for a different one. That makes it more difficult for the seller as they have to actually swap the case then change the listing for the other bass. I'd tell you "no", too.