It varies by hatchery but most will give you store credit either the difference in price between pullet and roo, or a full store credit for that chick, some will actually refund you the money so read their policy or ask them... Pay attention their guarantee as well, if they guarantee 90%...
Overall I believe that all the major big name hatcheries are decent and comparible, you are going to find good and bad reviews for all... My advice is choose the closest one unless they don't have what you want...
If you want the best selection you really have to start ordering as soon as they offer pre-sales, for many Hatcheries this is all the way back in November/December...
This isn't entirely true, local farm stores order from mostly the same hatcheries online that the rest of us might order from, and many farm stores offer 'special' order forms giving you options like Merek's vaccination if you feel you need as well as offering breeds they won't 'stock' for their...