Car stereo guy here: I think it’s reasonable for manufacturers to charge for an app, since anything that allows tracking and cell phone control of your vehicle outside of Bluetooth is always subscription based. If it was a feature inside the car or off a regular remote control transmitter then that would be completely different. If the app is so poorly designed that it won’t work if you have half a tank of fuel or less, or if it seems prohibitively expensive like more than maybe $100 a year )and includes tracking), then check out car stereo shops in your area. There’s a company called Mid city engineering, there’s Compustar, and there is DEI/Viper, one or all may have a remote start product for your Mercedes. One time cost. If you want something that is app based they will have options then you will be back to a subscription, but it might be better value.