When it comes to fees for checking a bicycle on a flight, Air Canada is quite reasonable (only $50 handling / service charge) if not a leader (charge waived for Altitude members). Our neighbours to the south -- less so. Both American and United charge $150 for bicycle handling.

Here's my question:

How do the "First Marketing Carrier" rules per US DOT Regulation 14 CFR 399.87 and CTA Regulation 144-A-2014 apply to bicycle handling / service charges, if at all?

Example:

Outbound
YYZ-ORD AC501
ORD-DSM AC5742 (Operated by United)

For a non-Altitude member, this would incur Air Canada's $50 handling charge plus applicable bag fees.

Return
DSM-ORD AC5838 (Operated by United)
ORD-YYZ AC512

Since the first flight is a codeshare, does the US DOT regulation apply to the bicycle handling charge resulting in the marketing carrier's (Air Canada's) $50 fee or does the operating carrier's (United's) $150 fee apply?

Appreciate the help.