Question: I have been contacted by Domain Registry of America to renew my Domain, but I think I originally registered with GlowHost through one of the sign up package options.

Answer: This is a pretty common question.

Your domains, if they were registered by GlowHost will indicate your "registration service provider" was GlowHost.com in the WHOIS results, and that your Registrar was either InstaPro.net, WildWest Domains, or most commonly, OnlineNic.

I'd throw the envelope in the paper shredder, they are sneakily trying to get you to transfer your domain to their domain service. I can't vouch for their level of service, but I can say their business tactics are shady, so if that's the case, why give them a single penny?