While I believe IB tries to charge unbundled commissions correctly, you cannot trust that they will. Some months ago NYSE changed their fees substantially. IB did not come close to correctly charging the new fee schedule, resulting in substantial overcharges. It took me 1-2 months of nagging to get them to acknowledge and fix the problem. There is a thread I started that discusses this in detail.
Before this I had naively thought that IB just passed on the cost as it was billed by the exchange, now I know that they attempt to emulate it.
My solution was to write a Perl script to calculate commissions and fees. Periodically I spot check individual trades to see if their commissions match what I expect. For me this works well because 99% of my orders are well defined as adding liquidity or MOO/MOC. I donââ¬â¢t have to worry too much about whether my limit order is taking or adding liquidity because they are placed long before the market reaches my price.
Of course this means that I have to keep up-to-date with exchange fee schedules, but they donââ¬â¢t change too often.