How do you calculate the count of bytes you sent on an AT modem ?

[Boris shtrasman]

I noted that many are using AT based modems to surf the net (aka NetStick)
and I wish to buy one.

Afaik the payment is per byte amount you sent/downloaded (and not the ip
datagram lengths).
And since in many places there are many retransmit due to packet and low
reseption I  wonder how I can overcome this problem :
counting the retransmits   and knowing how much you sent/got.

I can't find any tool than can give this info (since as I understand ethtool
will  not count what actually sent using the modem but only the ip datagrams
that are sent (much less).
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