They're designed for Cash > PayPal.
There's a 5$ usage fee which you pay for at the cash register.
You can only pay in cash for MoneyPak.
You must have a bank account attached to your PayPal in order to redeem it.
You can buy them nearly anywhere. Some places don't sell them. Sarasota, Florida has banned them from being sold in CVS for example. However, you can use Cash > PayPal which is basically the exact same as MoneyPak with a different name.