This will exchange 99% of the fluid vs. the normal "transmission service"(which is only a filter change and 5 qts of ATF) and also if they do a torque converter drain, you're still only going to get maybe 80% Mercon 5.
Do it right or don't do it at all, because you'll just be wasting your money throwing 5-8 quarts of M5 into a 14 quart system. Not only are you diluting the better M5 ATF, but you're mixing it with the leftover burnt fluid.