It seems like every few months someone starts a thread about an outside company becoming involved in renewals. Some send official looking letters. I got one this year from the "Maritime Documentation Center".
If you start at: https://www.uscg.mil/
there's a pull down menu that directs you to the real US National Vessel Documentation Center.
New to me this year was the ability to pay with a debit card instead of having Pay.gov account. It took less than 5 minutes to renew.
There are legitimate companies that take care of documentation for people w/o the time. But you pay a fee beyond the $26. Most offer a service to automatically pay your renewal. If you don't mind the fee. But if your renewal is paid more than 60 days in advance your new documents exp date will be shortened by that amount.