Even if UA wanted to not collecte their published rates, they would still have to pay the partner for transporting the bike. They should collect the fee, and issue a "two-leg excess baggage" coupon so that the receiving carrier doesn't charge you a 2nd time (It's like a ticket for multiple carriers, only for excess baggage.) When you get to the 2nd carrier, you give them their coupon so they can submit it to ARC to get their share of the fee.