OnStar data plan question

I've had my Volt for almost 2 months...I'd received one notification that my data was running out so I bought some more for something like $15 and now that's run out too....I'm the only one using it and I don't think I use all that much. I told them not to renew it again, and we'll see how that goes. Am I just double paying for data from OnStar as well as my phone provider as I've got an included amount of data with my phone anyway for when I'm not in a wifi hotspot. It seems to me that I can get by just fine without any wifi on in my car. Anyone else have thoughts on this?