You do get paid - for the most part. No company should ever ask you to pay them to work for them. There are plenty of legitimate mystery shopping companies that do not charge. Something smells of scam if you have to pay the company. Now the shop, you will have to do out of pocket & wait for the reimbursement.
Unless, of course you are doing a product evaluation. Then you may have to put down a deposit for the product, just so you don't accept the product & never complete your end of the bargain. Hope this helps!
Oh, and NSite is FABULOUS!! They pay you the end of the month in which you completed your shops. I have done one on the 27th, and got my paypal payment on the 31st. They are great people to work with, as well. One caveat - you better be good at what you do, because they don't accept any 2 sentance narrative or BS submissions. No shopper fee for some things either, but they reimburse well. I mean, $40 - $70 for dinner, $25 reimbursement at the liquor store...can't beat it!