I'm sure this won't be news for most of you, but I discovered this trick by accident. I'd noticed that KLM (KL) imposes a €10 “booking fee” on all online transactions. You can avoid this simply by performing your online booking on Delta (DL) instead as they have codeshare ventures between them.
I haven't checked for intra-EU methods yet, I'll slowly work my way through that (unless a reader wants to share the tip in the comments below!).
Here's a screenshot of DL's booking page for an AMS-YUL round trip codeshared by KL/DL but operated by KL:
For comparison here's the total price on KL's booking page for the same flights:
and when you click into the “taxes, surcharges…” link:
Since both price in the currency of your departure city, there's no need to worry about foreign exchange discrepancies between the two, bar a penny or two.
As an educated guess, in order to book on DL, I imagine there's a minimum requirement of at least 1 segment marketed (but not necessarily operated) by DL. I'm not going to bother testing this hypothesis though!
Since both use the same booking engine (Global Distribution System — GDS) you can easily access your reservation on each airline's webpages to change seats, order special meals etc. I've had to do exactly this for an upcoming reservation which uses both KL and DL, thus my accidental discovery.
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