Hi Loraxc,
There are usually some charities or non profits in most areas of the country that will take a lightly used mattress and if you wish to return a mattress that you purchased from a company that allows you to donate your mattress instead of shipping it back then they will usually help you find a local charity (if there is one) that will accept it and if there isn’t one then they will let you know what to do to receive a refund.
For those who are looking to donate, recycle, or dispose of a mattress that is more than “lightly used” then some of the information in this topic may be helpful.
Phoenix