Yep, up until we perform the assignment, they have the option to change the terms of the job. They can cancel it, just as we can cancel it, and they can change the fee just as we can call and say, "I know I accepted this job for $5, but I really think it's worth $10 now, and I won't do it for $5".
If we cancelled or called to ask for more money, they have a right to be annoyed and reconsider whether they want to keep doing jobs with us in the future. Just like we have that option when the reverse is true.
This seemed like an honest mistake that was fixed quickly and everyone seems to have been notified of the error properly. As bummed as I am to not get a chance to do super simple shop for hundreds of dollars, I think I'm filing this one under "s&@! happens" and moving on.
Shopper in California's Bay Area