Instruct: Classify a given review into positive or negative sentiment.
Query: Too thick

This is advertised as "low profile" but the first time I put it in my mouth I knew there was no way I could wear it. The outside of the bite portion is way too thick and rubbed on my gums. The paper inside the box said I could return it for a full refund so I sent it back certified mail per their instructions. When I traced it through the post office it said it was "undeliverable". I am very disappointed.