That is simply standard language and not particularly applicable to your e-ticket presuming that you are connecting at ORD and flying onwards to ATW. If that is the case, your bag will be checked through to ATW at LHR. You will collect it for CBP processing at ORD and then drop it off with AA after CBP. But, there will not be any additional fees.
If not a connection, then the AA domestic bag overweight fee is $100 per bag.
Incidentally, your TA ought to be able to generate and have emailed an e-ticket receipt compliant with US law.