Some banks will hold back your charges and apply your account balance to the largest charges coming in first, or the ones of their preference. This is usually so that if you have a low balance, they can get you for an extra fee on each of the smaller amounts instead of the one larger amount.
But if you are able to get one without being charged, consider it a gift. Walmart screws over all of their employees. In many cases they keep people on at 30 hours so they don't have to pay their pitiful benefit package. I think that people should be educating themselves and finding better employment. Also the government should be forcing them to provide proper benefits.