Whenever you are given a time range like this for something like a shuttle, it means that they expect to arrive to pick you up somewhere within that range - and thus you should be ready before the initial time stated.
As well as being common sense, this is also clearly stated on the Supershuttle Website
The vehicle should arrive within the 15-minute window selected at the
time of booking. It is the customer’s responsibility to be ready and
waiting to depart outside of the pickup location at the beginning of
the 15-minute window, and remain for the full designated 15-minute
pickup window. Failure to do so may inhibit our ability to provide
service and may result in possible forfeiture of fare.
Their website also states that no refund is due if you are a "no show", which is a fairly standard condition of services like this :
SuperShuttle will not provide a refund if the reservation is cancelled
or changed less than 2 hours prior to pick-up time, or in the event of
However they also claim that they will attempt to contact you using the phone number you provided :
Reservation abandonment occurs when the Operator cannot find the
customer at the designated pick-up location and cannot reach the
customer for further instruction using the “Day of Travel” contact
number provided within a short, reasonable time period, so as not to
jeopardize pick-up of other shared-ride passengers.
If you did provide a valid phone number, and if they did not attempt to contact you on this number, then you might have a valid claim for a refund based on that fact.