What this shop is referring to is often times called "power flushing". Many places that offers such serves require that you first sign a waiver releasing them from responsibility.
Because this approach has a tendency to loosen dirty that may be in the system. A spec smaller than you can imagine can deep-six your tranny.
There's a defined approach to changing MB tranny fluid and filter. In most cases it involves draining the converter, main pan; replacing the filter. Hire an garage versed in MB repairs/services.