I don't have Reagan's autopen styles nearby, but a letter that starts "My Fellow American" or even personalized and is any sort of a marketing letter is a mass-produced letter that virtually only comes preprinted or as one signed by a mechanical signing machine (Autopen).
This is almost certainly NOT authentically signed by Reagan but I cannot tell for sure whether it is autopen or printed. Although I do not see an image of the entire piece, the greeting “(my) fellow American” virtually guarantees that this was a generic item, mass produced—not the kind that Reagan would hand sign. Sorry.