I've been doing this for several years and I think I can shed some light on whey you had to pay for your bag. Your free bag on Delta is only when you book on Delta with your Delta Amex card. You will not get the free bag is you book on Expedia, not even is you book on the KLM or Air France site. It does not matter who operates the flight, as long as the flight has a code share for delta, you won't be charged. Or if you have a flight first or last that does not have a code share, but they issue bolt fares, the luggage allowance will be based on the international, or transatlantic, or transpacific portion of your flight. So if you fly from SFO-AMS on a KLM, DL codeshare and you connect to ZRH from AMS then your luggage is checked through and you won't pay if the SFO-AMS is a codeshare flight operated by KLM. But your ticket cannot be booked through anyone else but Delta. When you fly back, then the longest flight determines your luggage allowance, so if you fly ZRH-AMS-SFO, then the AMS-SFO portion determines your eligibility. It's a matter of knowing the rules and your rights. As soon as you book on Expedia, you are traveling by Expedia rules and those are not great. I hope this helps.