10 days is a lot and could be a problem.

Payment not guaranteed is a problem.

About the 10 days deadline,

first of all, a shorter deadline will get the best designers working for you as soon as possible, a long deadline would just make them postpone again and again until they forget about it.

Second, designs will come in a lot quicker when the deadline is closing in.

And third, a week from now, we would hardly find your contest, it would be on the 10th page or something like that, that can't be good.

It's your decision but i assume you want the job done as soon and as good as possible.

Thanks for the advice. Doesn't look like I can shorten a project at this point.

You can, it's been done before by many others. I don't know how though, but it shouldn't be too hard.