We have tracfones and use the codes almost any time we re-load minutes. We get codes via e-mail from tracfone and on the tracfone calendar they send out. Be aware,though, you can only use a particular code for one phone, even though they generally don't say they are one-time use.
Also, we had a problem with tracfone putting extra minutes on our phones when we would re-load minutes on their website. We didn't think it was a problem at the time. We thought tracfone was simply rewarding it's customers with extra free bonus minutes, but apparently, sometimes tracfone's own system adds extra minutes "by mistake" and when they discover the error, they will take the minutes away, disable your phone, and not allow you to reactivate your phone. They will also accuse you of somehow adding "unauthorized minutes" to your phone through their website (as if we are all computer hackers who know how to fraudulently do things like that-yeah right). So, you end up having to purchase a new phone and you lose any 'double minutes for life' or 'one-year' cards you had on the old phone.
When that happened on our son's phone, we switched his to T-mobile prepaid and are still waiting for tracfone to disable the rest of our phones (because we've received these "unauthorized minutes" several times on each of our phones).
Just beware that apparently tracfone does not ever give out more free minutes than is stated on the coupon. They don't give extra rewards for using their website and they will not put a disclaimer on their product that free minutes are not always actually yours to use. (I did inform tracfone that we would be telling everyone we saw using a tracfone what they did)
Hope you never have this kind of trouble with yours!