Yes, technically they can. Nobody is stopping me as a software engineer from showing you a different price to what I'll eventually command the payment processor to take from your bank account.
You are safeguarded if 2FA or 3D Secure is triggered, because the bank would send you an authorisation code you need to enter to complete the transaction. The message containing the code would also state the amount that I plan to take from you.
To answer your question, technically it's possible. The card processor would drop me in no time as @littleadv stated above and I would be legally liable but it's still technically possible.
Just to be clear; @littleadv criticisms in the comments below are correct. You're asking 2 different questions and I'm only answering one of them.
It's not the same as giving your merchant your wallet even though it is technically possible to charge you whatever amount I want regardless of what I'm displaying. I'm only answering the question: "...merchant technically can charge whatever amounts he wants regardless of "online website cart amount" when confirming payment?"
The answer to that is: yes I can.