The Domain Registry of America is a scam organisation which fraudulently invoices individuals and companies alike for renewal of .com/.net/.org domain names. They work on the assumption that clueless people will simply fill out the form and send it back - but by doing so, you end up paying far above the going rate for the domain name renewals, plus they actually take ownership of your domain names and move them away from your current registrar.
How do they send you convincingly-written letters? Well, they just poll the public whois information for your domain names (which includes expiry date, full address and full name). So, be mindful also that whatever information you provided during registration will be available for the whole world to see! You may consider anonymising some of the data, or going through a third party anonymous registrar service (which will cost extra on top of the domain registration fees - GoDaddy currently charge $20 for two years).
The Domain Registry of America is one of the older scam organisations making money off the backs of unsuspecting Internet users, but sadly their 'business' persists. UK-Cheapest has published an article detailing the DROA and their various other companies (with similar names) - read and avoid if you get a letter through the post from them!