It can be done with the oil pan installed( have someone hold a vac to catch/pull the metal out and drill slowly,to bad your way over there, but realy if you can drill a straight hole then it's not that hard,the tap and all come in a kit. Check out some U tube vid's and get a look for yourself how it's done.
O and if that shop dosen't doo anysort of heli coil replacement I would find a better shop to deal with for future repairs.They are obivisouly not well equipted or educated.
LOL equipted ..the pain killers at work I will shut up now.