In the past, Maritz allowed self-assign privileges after a shopper completed several (don't know the magic #) shops successfully. I assume they are still following that method. You did the right thing by emailing for them. I bet once you've completed the shops, you'll be able to self assign in the future. As for the dates, there may be rotation issues with the dates. They need a certain amount of time between last month and current so if the shops were done late in the month, earlier dates might not be available. Of course, I'm just guessing about this. I'm not sure why you're seeing it that way. I seem to remember being able to delete a shop before completing the request if I no longer was interested in it, unless something has changed. There should be a box to check "No longer interested" or something similar.
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The more I learn about people...the more I like my dog..
Mark Twain