A couple of thoughts.... First, in this case, we are talking about $14. (Yes, there is a bigger picture, but, however this issue is resolved - or not - we are only talking about $14...)
It is completely unreasonable to expect us to check our email every hour. You had no reasonable way to know that the shop was cancelled when the email was sent. Had you been on a drive to a far-off location, you would really have been burned, with the email coming after you had departed for the trip. This is the sort of thing that would cause shoppers to stop working for an MSC.
Try writing a respectful email to the project manager explaining the situation and asking that the decision not to pay you be reconsidered. Recognize that the email was sent an hour before you started the shop, but point out that you had checked your email before the notice had been sent and had already begun your day of shopping and could not stay connected to email while performing shops.
There is no guarantee that this will work - it may depend on who the project manager is. However, just as you are only talking about $14, so are they. Would they really risk losing a good shopper over $14? I know that if they were to start doing this with any sort of frequency, I would hesitate to shop for them - even if I were not directly affected.
If the email to the project manager does not work out, you could go up the ladder and find somebody higher up in the company. However, even if the higher-up were to agree with you, you might upset the project manager and schedulers in the process, which could make getting future assignments more difficult. So, you will have to weigh your options. While I agree that you should be paid and this sort of thing should not be tolerated from MSC's, I am not so sure you'd want to put your neck on the line over this by going up the ladder. Remember that project managers and schedulers change companies with some frequency, so you could find yourself sticking up for yourself here, inadvertently hurting yourself elsewhere.
Hard work builds character and homework is good for your soul.