You need more than just a booking confirmation. I assume you are staying at one of the Marriott's since you are posting here. Here is the link . . . .

http://www.visitmoscow.ru/english/moscowguide/

(click on the visa section)

If you email them, they will email back the form containing the information they need to complete the paper work. When you complete this, they will send you the voucher and and room confirmation. You need both of these to apply for the visa. If this is the only place you are staying, you are fine, they will take care of everything. If you are staying other places, Marriott will only provide what you need for the dates you are staying with them. In that case, you are better off to have one of the other hotels (if not Marriott) provide this as they will usually do it for the entire stay. You don't need to have separate ones from each hotel, but you do need to be covered for the entire stay.

Marriott does not charge for this as long as you go and stay there, however, if you cancel, they will charge you. They will ask for a cc when you send in the papers. They will then fax the completed invitation and you include this with your visa app.

Also, the Marriott Royal Aurora is the closest to Red Square and by far the nicest of the Moscow Marriotts. Try to stay there.

PM me if you have more questions. There is no need to pay for someone to prepare your visa app, it is not as bad as people make it out to be.

p.s if you need a guide let me know, I have two I used a few times and they are very good and reliable. Also, if you are paying for your room, try to find a rate that includes breakfast unless you can do without it. For example, the breakfasts in the Royal Aurura are over $60.00 for the buffet + tip!