Didn't know all this. The only time we've ever been charged when going over our data limit is while we were someplace that didn't have wifi, especially while gate guarding, when we tend to spend a lot of time on the net out of boredom. As far as downloading updates while offline, again I don't know of that ever happening. Sally has gone for months without updating software on her iPhone, while I'll usually update mine, unless we are on the road, & I'll wait until we've got a good steady wifi, or just wait until we get home. BTW we do have AT&T, 2 iPhones, plus 1 iPad.
Thanks for the heads up, will have to start watching closer.