It is $15 per 1GB that you go over your limit. Verizon does not slow down the speeds that you have access to.
You can also change your plan to a higher one to give you more data. This can be done by logging in to the website.
Make sure that you BACK DATE your change to take effect at the beginning of your current bill cycle. This way you will save a little money.
We're always happy to answer any questions you have and are excited to welcome you into the Verizon family. To answer, if going over your data allowance an overage charge of $15 is charged per GB of data over. So if your allowance is 12GB and for your billing cycle you have used 14GB of data the total overage cost will be $30. We do always keep our customers updated with notifications of usage and always offer the options of upgrading their data plan to avoid the overage charges. Let us know if this information helps.
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Just to be clear, it is $15 per any portion of a GB you go over. So if you have 12 GB and you use 12.001 GB, you will get the $15 charge.