if you buy a booster pack make sure its made by some proper reliable manufacturer, To be honest you can just buy an SLA battery and solder some cable and clamps on it. The pack has exactly that in it.
Just recharge it every month or so.
To be honest - nothing beats a set of jumper cables abt 26 sqmm close to 2AWG and real solid copper jaw clamps, You should also know where to jump it from. In mercedes benz you have the remote terminals in the engine room but they require that you connect the jumper cables and let the car's own battery charge up for about 10 minutes before you fire the starter from the key - also make sure the smart key is NEVER in the ignition of your benz when you are hooking up any cables