@neoasr - rather than just saying 'no thanks' or similar, it would be more helpful if you told us what you didn't like about people's suggestions.
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it really should be down to the isp to provide some means of keeping track of usage, either through providing some online meter on their website, or providing a decent modem/router which keeps count.
but i suppose they profit when you 'go over', so they don't have much incentive...
an old isp used to send me an email when i hit 80% of my limit - luckily now i can eat as much as i want.