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.
Primary: iPhone 6 64GB, Galaxy Note 3, Lumia 635: 5 lines, 3x $5 Android web, 1x $2.99 T-Zones, corp discount + $10/mo loyalty discount = $93/mo tax inclusive
Secondary: iPhone 4S & LG Ultimate 2: TracFone (Verizon)
Tablet: iPad Mini Retina w/ T-Mobile 4G LTE ($10/mo for 5GB with hotspot)