I'm afraid that sounds just too cheap to me and I'd be wondering what they did/changed....

Pretty certain that all W212 models use the latest 7G type gearbox which takes a minimum of 5L of ATF (if you don't empty the torque converter) and 9L if you do a FULL ATF change, plus the filter and sump gasket have to be changed as well - at £150 it sounds like it would just cover parts alone. I wonder if they also used MB STAR or an equivalent that is critical in the process to check the final ATF volume is correct - the 7G box has no dipstick to check the level manually, so without the correct diagnostic you're just guessing the level is correct.

Perhaps they did do everything as they should have, but at that price i'd be checking - cheapest isn't always best, especially with Mercedes.