87% of wedding guests buy outfits they only wear once. ThredUp's new pop-up and dress code decoder offer a cheaper, faster way to dress for the occasion. The wedding-industrial complex has always extracted plenty from the couple, but a quieter cost has been piling up in guests' closets. According to new ThredUp data , 87% of wedding guests have bought an outfit they wore exactly once, and 68% of those pieces are still hanging in closets, waiting for an occasion that never repeats. The culprit is a feedback loop of hyper-specific dress codes ("Amalfi chic," "old Hollywood glamour," "colorful cocktail") and a social-media norm that treats outfit repeating as a minor public failure. Nearly a third of guests say they can't reliably decode what these invitations are even asking for. On May 30, ThredUp is turning that anxiety into a storefront.…