I have used custom-made orthotics from a podiatrist for about 5 years. That is until Medicare became my primary health insurance. These $400 orthotics are no longer covered. The doctor had given me shots periodically but you cannot go on doing that forever. I tried various drug store sold shoe inserts to no avail. So, I looked around on Google and found the Protalus site.
Given they offered free return if the orthotics do not work I purchased a pair of T-100. They worked, but not 100% for me. I called customer service and asked if they would exchange the T-100 for the M-100 and they did that with me receiving the replacements in about 3 business days. I have been wearing the M-100 in my Merrell slip-on shoes for about a week and they are a keeper. There is no (thankfully) heel pain. Today, I put in an order for 2 more pairs of the M-100 so I don't have to move them around from shoe to shoe..