Unfortunately, there isn't an offset property for the mobile toolbar. However, you can position the toolbar at the top of the screen rather than the bottom using the following (notice the layer key is "mobile" not "share"):
addthis.layers({
share:{},
mobile:{
buttonBarPosition: 'top'
}
});
To offset the toolbar, you can override the css with the following:
.at4m-dock-toggle { bottom:50px !important; }
#at4m-dock { bottom:50px !important; }