There are no codes for extra minutes in the way you are thinking. Their computers generate airtime codes based on your ESN/IMEI + SIM serial; they are unique and one time use only.
Castrol EDGE 5W-30 is my drug of choice.
Toyota | let's go places
Primary: iPhone 5S and Galaxy Express (T-Zones line): 5 lines, 1x $15 4.5GB data, 3x $2.99 T-Zones (truly unlimited 4G data), myFaves, corp discount + $10/mo loyalty discount = $89/mo tax inclusive
Secondary: Galaxy Fascinate: TracFone BYOP (CDMA - Verizon); ZTE Avail II: Airvoice
Tablet: iPad Mini