For what it's worth, KLM is selling returns on the INV-AMS-ORD route next month starting from £436.19 per person in Economy Light. On those same flights, Economy Standard comes in at £556.19 per person; as such, at an absolute minimum, you are looking at an absolute minimum of £120 to check a single 23kg bag - that "Standard" economy ticket is £60 more than the "Light" ticket each way.

However, my suspicion is that it will be much more than this; it's always cheaper to buy luggage in advance (preferably as part of your ticket) than adding it on the day of departure at the airport; and yes, I realise it is not "fair" that it seems that you have no option of pre-purchasing your baggage in advance, but that is a complication arising from the way the ticket was purchased/issued.

NetFlights offers the same flights for £451.19, so it looks like they have added a £15 fee of their own on top. For a fare including a checked bag, they offer these flights at £562.19 (which represent only an extra £6 on what KLM themselves charge)

At these prices, it would have been advisable to book directly with the airline operating the flights. However, in any case where you want to make even a minor modification to what has been booked, it's always better to have booked directly with the airline. Having had some frustrating refund experiences with tickets booked via some of the biggest and best-known online travel agencies during COVID, I have now completely sworn off booking using third parties. Only if there was a *substantial* saving to be made would I now consider booking via an online travel agency