Often these are akin to the telemarketing calls to switch your long distance plan.  A majority of the time the domain name in question is not up for renewal, or they are listing a domain name you currently do not own with a different extension.  If you own xyz.com they may be requesting payment for xzy.net.

The best way to make sure you can toss this letter in the trash is to verify your domain's experation date.  To do this visit: http://www.networksolutions.com/whois/index.jsp  then enter your domain name in the window and press search.  This will return a whois result for your domain name.  Look for the experation date to verify that it is not soon.  If you would like to renew your domain name simply, or have questions please open a support ticket and we will be happy to help.