Please note that the $100 r/t price is for a town car service.
Most companies offer both limos and town cars. The typical price for a limo is about $200, which ends up totalling about $250 by the time you add in fees, charges, tips, etc.
It seems like the most common carriers that people on this board have used are:
www.fltours.com
www.happylimo.com
www.tiffanytowncar.com
If you are doing a grocery stop, the total price from any of those three companies will be pretty darn close. Make sure you add up all the fees and extras and see if the tip is included or not (some include the tip on limo rides, others do not).