That is very frustrating. The longer I did this, the more I learned.
1 - What a "mass" email looks like. 99.9% of them are mass emails, even if they don't "look" it. The email that says, "Hello Matthew, This is Joe, the scheduler for Shop XYZ. I see you have done this shop in the past for us, and did a great job. I was wondering if you might be interested in doing it again, we really need this shop completed. Thanks, Joe." This is ALMOST surely still a Mass email, sent to every single person that has done this shop before, and using the BCC so you can't see anyone who the shop was directed to except you. A shop that says, "This is being sent to just you," "Please reply with a positive OR negative answer," or some shop with a specific thing about YOU is usually the only personal emails you get with a cold shop request. (This isn't always true, but a pretty good rule of thumb!)
2 - How to get that urgent shop. Send an email. If you don't hear back within an hour or two, try calling the mystery shopping company. Tell them you'd really like to do this shop, but need to move mountains to do it, and don't want to move the mountain and learn you're supposed to be on the other continent!