I think double-minutes is tied to the phone. I own a CDMA TracFone that was deactivated for just under six months. When I reactivated it and added airtime, it still provides double-minutes.
TracFone models with a C in them, are CDMA phones and (with TracFone) will activate on Verizon. Models with a G in them are GSM/HSDPA and will activate on AT&T or T-Mobile. When looking at a GSM phone package at retail (e.g. Walmart), if there is a P4 on the white label on the bottom of the package, that phone will activate on AT&T. If it has P5, it will activate on T-Mobile.
You will not be roaming as long as the phone is on the same carrier it was activated on, when you are traveling. My understanding is that the agreements TracFone has with the carriers, would ensure that the phone will pretty much work everywhere in the U.S. where there is coverage, roaming or native/home network. More recent phone models (both CDMA and GSM) will display "Home" when you are on the home network, even with outside your home area, and will display "Roaming" when the phone is roaming on another carrier's network. Older phones displayed "TracFone" as the network when you're on the home carrier and "Roaming" when roaming on another. There is no extra charge when roaming, the phone deducts 1.0 minute for every minute on a call for voice calls. With the newer models, if you want to use data/web browse - if the phone includes a browser, the phone deducts minutes based on the actual data usage. Text messages cost 0.3 minutes for each message sent or received. I have not yet sent a picture message with the new phone I have, so I do not know what the new charge is for picture messaging.
The zip code is requested so they can provide the actual phone number for the activation, which may or may not be local to you. Several years ago when I activated my first TracFone, I used my home zip code and ended up with a phone number that was 2/3rds of the way across Massachusetts. Although it was in the same area code, it was not a local number, not even close. Perhaps they have fine-tuned this since then?'
Hope this information helps.
T-Mobile customer since 2005.