I recently had a miles+copay upgrade into CDG airport clear, and found a $48 charge from United on my credit card for an airport passenger duty tax. I'd heard they were doing this to LHR, but didn't know it also applied to CDG. Of course I'm annoyed at this, given that I already paid a hefty copay, but my question isn't about United's decision to impose the fee, but rather its disclosure or lack thereof.

I'm generally pretty good about reading what I'm clicking through, and I don't remember ever seeing any kind of disclosure about this $48. It annoys me even more when people charge stuff to my credit card without first telling me the amount or letting me chose the credit card. Can someone who is familiar with the online upgrade process recently tell me where this amount is disclosed, and how to know whether and how much I will be charged in addition to the stated amount in the future?

Thanks.