I count both packets and beans, and as a customer (though not Barb's), I
think such a charge is entirely within reason -- because it is under my
control. Have some more cake.
ISPs bill customers for traffic on the edge. If you filter one hop from
the edge (interior of the edge router - fewer interfaces that way too) or
at your border, then you can have your cake (money from the customer) and
eat it too (filter RFC1918). Of course you would then be charging
customers for packets you don't pass. They'll never know, and I never
met a bean counter that cared about such details anyway... if bean
counters are making routing policies.