its not a ‘problem’, but the way that united enforces point of sale. Most airlines do this by simply using the country of the departure city, charging in that local currency, and allowing you to use a card from virtually any country. UA does it by billing address. Not sure why, or what the benefit of doing it that way is - there must be some for UA to continue to use it. But it does bring exactly the problem described by the OP, which means the prices can change, more than just based on exchange.not sure the relevance of this. OP wants to pay with a Canadian price on a US card. If you start searching on United.ca and enter United States in the billing address, will reload on United.com with in USD using prices on US POS and only accept a US card. Google flights will route you to .ca based on the YYZ departure, but UA will take you back to the US site when trying to use a US card, and price in US. Standard UA website behavior. In fact, you are having essentially the same thing done to you that OP is having done to them, except they want the Canadian price that saves them ~$10. UAs site does t allow it, probably because they don’t want the OP doing what hey are trying to do.

OTAs aren’t leaving processing to the airlines so the passenger gets more points - they are doing it to save $. When United (or whatever carrier) processes the charge, they pay the card processing fees. Instead of the agent. So they will let the airlines process it every time. Airlines still allow this, since it’s an incentive for the agents to use their carrrier (guess what would happen if AA tells Expedia they will process, and UA says they have to do it themselves...who is the agent going to try to convince you to book with).