In the past week, two clients have contacted me to ask if they should pay a domain name renewal that has been sent to them in the mail.
Please be aware that there are some dodgy companys around that will send these letters, hoping that you will pay their inflated prices without thinking about it. This practice is apparently not illegal, although it's highly unethical.
Generally when I build a website for a client I also offer to take on the domain name management, as the company I lease hosting space from also offers very low-priced domain names, and it is also simpler to have your domain name and webhosting in the same place.
If you receive one of the letters, please feel free to contact me before you pay - in both of the aforementioned instances I was already managing the client's domain name, and both had many months left before renewal would be necessary.