I am thinking about adding an existing AT&T model iPad Air to my AT&T mobile share account. Do I need to sign a 2-year contract to get the $100 bill credit or just keep the device on my account for at least 45 days. The AT&T website http://www.att.com/shop/wireless/100-dollar-credit.html#fbid=pwNrpbp8JnV does not really say. In addition, will there be an activation fee to add it?