No need to prepay.
If the daily rate is higher than the Delta.com rate, just connect to gogo, open your browser and go to Delta.com (don't login to gogo, go straight to Delta.com) and buy it then.
Usually I just need to buy a couple 30 minute mobile plans (I change my browser to only buy mobile plans) which are usually $2 so I don't buy on Delta. If I'm flying east coast to west coast I buy the Delta.com one and save $$.