In 1 1/2-quart glass or stainless-steel bowl, combine shrimp, 1 cup lime juice and 1/2 tsp salt (add more lime juice, if needed, to cover shrimp and allow it to float freely; this will ensure that the shrimp “cooks” evenly throughout). Cover and refrigerate for 3 to 3 1/2 hours or until shrimp becomes opaque right through to center and slightly firm. Drain in colander.
Stir together mango, cucumber, red onion, jalapeño, cilantro, remaining 1/4 cup lime juice and salt; toss with shrimp.
Spoon ceviche over tostadas. Sprinkle with tajin seasoning. Serve with lime wedges.