If you cancel before they have accepted the shop offer, then there is no meeting of the minds and the contract does not exist.
It's not flaking and should not be held against you for retracting an offer that was not accepted.
I always try to get the scheduler on the phone and get a yes or no if another assignment comes up that I could take that conflicts with an applied for shop.
If I find a better assignment, I have no problem backing out of the application. If the scheduler really wanted you to do the job, they would accept the offer within a reasonable amount of time. A good scheduler will assign it within 24 hours on a work day.
To leave you hanging and tying up your schedule without accepting the job offer means they risk loosing your offer to do the job.
To leave you hanging in the applied status while they have another shopper do the job and have you sit in this applied status as a "backup shopper" is just immoral and I would think twice before applying for shops with an MSC that would leave a job in pending status for several work days.
This is where the Shopmetrics software shines. They have a "Backup Shopper" option that you can apply to be the fall back shopper. You know that your not getting the job unless the first shopper flakes.
Sassie on the other hand makes it as simple as one click to retract your offer to accept an assignment.