Hi Alex! Strip charges 2.9% + $0.03 for most debit or credit card payments (plus extra for currency exchange or international cards). Stripe also supports ACH (eCheck) payments, which all bank accounts support, with a fee of 0.8% or $5.00, whichever is less. If the organization’s Stripe account already has ACH allowed, this should be no trouble for them.
Givewell asks to receive donations under $1000 through card payment and over $1000 through check. However, not all charities are Givewell—some are happy to receive a check at $500, for instance.
Lastly, don’t forget to check to see if you can use a workplace charity matching program, as many of them cover credit card fees in addition to matching your donation.
I think there are a lot of factors here. If you’re a CEO in a wheelchair, you’re certainly better off working and donating. A low-paid person who’s physically strong might be very valuable in Ukraine.
I doubt you could really estimate the utility you can’t really predict the future—Ukraine could win and we could still have many of the bad outcomes people are scared of (e.g. nuclear war, Russia attacking Europe).
There are a lot of other things you could do to promote peace—like advocating policies in the US like renewing New START and reducing the “defense” budget (or equivalent policies in your country), getting a job in arms control/foreign policy (e.g. IAEA, Center for Arms Control and Nonproliferation), or focusing on understudied/funded issues like landmine nonproliferation.