ReRoute turns "sold out" into "sold" by automatically showing customers relevant alternatives when products are unavailable, discontinued, or upgraded. Unlike basic "related products" widgets, ReRoute lets you define why an alternative is shown — out of stock, newer model, own brand, discontinued, or similar — so customers get context, not just suggestions. Supports manual mappings, Shopify's Recommendations API, or both. Inline blocks or popup overlays. Free tier included.