From this date forward you can use iptables to keep track of your bandwidth totals.
The thing is, I believe that Mint uses ufw as a front-end to iptables, and I don't know how to save the counters using ufw.
First you need to configure iptables how you want it then save the rules to a file, for example /etc/iptables.rules
Then you need to set it up so they load at boot and save at shutdown.
One way you could restore your rules and counters is with a script in rc.local
/sbin/iptables-restore -c < /etc/iptables.rules
or where you want to save your rules and you could put this file in /etc/init.d/rc.local making sure it is executable
and you could put another script in /etc/rcS.d/S50iptables with
/sbin/iptables-save -c > /etc/iptables.rules
To save your counters and rules at shutdown.
You could check this out too