Your shop rate is what you charge the customer wanting the seat done right? So in our example here (allowing for speculation because we don't have specifics), the customer would be charged $100, and the internal labor costs would be $50 (because we were told there were 2 seats). So a 50% profit margin. However, that doesn't take into account any of the fixed costs of running the business or maintaining the machines. I also assume that this work has some amount of seasonality to it so having consistent work may be difficult, meaning larger fixed costs to cover time between paying jobs.
Does that sound right?
I'd be very tempted to install Firefox in my local appdata folders (which doesn't require admin rights to install), then install a theme to make FF look like Edge with something like this..
Still use real Edge browser for work stuff, but FF for less-than-work stuff.