Chop the shrimp and crab very fine. (I use a small electric chopper). Dice the tomato into small pieces. Chop the cilantro and onion into small pieces. Combine these ingredients in a large bowl.
Squeeze the limes into the mixture. Add salt to taste. Refrigerate over night or at least 4 hours to allow the ingredients to soak up the lime juice.
Before you serve, slice avocado like orange slices and arrange a circle in the center. Use 1 tostada per serving and spread Kraft Mayonnaise or Miracle Whip over 1 side of the tostada. Then spread the Ceviche over the mayonnaised tostada. Enjoy!