The discount is likely based on a bulk volume agreement to order a certain number of shirts even if all of those shirts aren't sold through the promotion. Adding more types of apparel after the contest and purchase window were defined could quickly destroy the budget set aside for this.

That said, all these designs are now owned by Bethesda Softworks LLC so there's really no harm showing interest in purchasing another design or style. But there would have to be a very strong interest to move to product design, and in that case meeting a preorder quota might be the most fair way to determine actual sales intent.