I just noticed this today while logged in to my Amazon.com account to check my order status. There was a notification about receiving a $5 reward if I reload my gift card balance with an amount of $100 or more.
Since I use Amazon to buy most of my stuff, this is pretty much a no-brainer for me so I went ahead with it. It looks like you can only use it once per account. You can use a credit card to reload your gift card balance so don't forget to use a credit card that offers rewards!
In case you don't see the notification under your account, you can try this link.