Every year we get a number of emails from clients about questionable-looking "invoices" for domain services. It is usually a very generic document that has what looks like an expiration date and pertains to their domain name. There are a number of different companies who send these out and they are mostly junk subscription offers that provide a service that our clients will never need.
This is an example of one:
I removed my client's domain on this one, but it was listed by DOMAIN NAME and by Website at the top. To the uninitiated, this looks like a request to renew a domain name. The letter states directly that it is not a renewal for the domain name, but a subscription service. For most people, though, this distinction is not clear. This particular service supposedly moves your domain up higher in search priority with services like Google and Bing. I can't speak to their effectiveness, but I do know that the website they put on their document doesn't work at all. The Better Business Bureau has a number of complaints about this particular company online.
In any case, this is not a domain name service that is necessary and should not be paid. Most of my clients send these requests to me so I can verify them before they pay them. It is important, though, to look at any of these types of documents closely if you get them. All of the registrars, from Godaddy to Network Solutions, try to upsell their products with services that the end-user simply does not need. Try to buy a new domain name on Network Solutions and you will go through about 4 pages clicking No Thanks to the extra offers.
If you want to know who is responsible for your domain name registration, go to http://whois.icann.org and search by your domain name. (I would search for opustechllc.com.) You will have to type in some captcha information to prove you are not a robot, then it will give you your information. Look for the Registrar information on the page. If the document you got is not from that Registrar, then someone is likely trying to get you to switch to them or it is a service subscription like the one above.