As a commercant I have had to recently register with RCS which, like many others, has put my business on a list which is available to others to use and abuse. When I first registered as an autoentrepreneur I got a wave of somewhat suspect letters all asking for money. Bewarned registering for RCS is no different.
One of the main culprits in this type of scamming is a company called Kbis info-services. Although the address on the top of the letter is from Montpellier, or in some cases from Nimes, they are actually based in Tunisa. The small print on the back is very well thought out and well written. It does in fact get smaller and smaller as read down the letter. This has been done presumably to stop anyone sueing them.
If you get a letter from this company you do not have to pay it. It is a scam!