A lot of the shops have a date range and the shop can be completed any time within that period. If that is the case you will see a reschedule button when you open the shop and it will give you the dates it is available to be shopped. My experience has been that it will only allow you to do this once so choose carefully.
All these posts are excellent! I also just wanted to add that recently (thus past Friday), I had a phone shop where I had to call and get info and then fill out my report and send it in for review. However, the location wasn't open that day due to weather conditions. What I did immediately was contact the MSP and let them know what happened, so I wouldn't be penalized for not completing the shop on time. I guess they made an internal note regarding this, and are allowing me to complete the shop tomorrow (Monday).