Orthotic sandals are a rather new trend. What they are is a sandal that doubles as an orthotic. Essentially, an orthotic is glued into the footbed as opposed to a traditional footbed. This gives you the advantage of wearing an orthotic while you wear sandals, which is not the easiest thing to do. Now finding the orthotic that is correct for your foot type is not easy, especially if you have flat feet. Flat feet have specific requirements. They need an orthotic or sandal that molds and contours to the shape of your foot and then provides solid and strong arch support from there. Below I have given a couple examples of orthotic sandals that I think are ideal for flat feet:
Spenco Kholo Orthotic Slide Sandals- These orthotics contain a material called EVA in their footbed. What EVA will do over time is mold and contour to the individual shape of your foot, and then provide arch support where it is needed. This sandal also has a deep heel cup which helps support stability as well as function as a counter lever to overpronation.
Vionic Orthaheel technology Wick Sandal– This sandal features an EVA footbed with a deep heel cup as well. The difference between this and the Kholo above is this probably provides stronger arch support while the Kholo will be more comfortable and provide a greater degree of shock absorption for your foot.
Vionic Orthaheel Technology Pamplona Supportive Slide– If you have tried other orthotic sandals with poor sucess consider trying a cork EVA sandal like the one below. Sometimes feet just respond very well to cork. It provides both shock absorption as well as arch support solidly.