I once worked on a reasonably complex e-commerce system with millions of products, add-to-cart, taxes, promo codes, etc. Every so often, an order total changed between the time that the user saw the total on the confirmation screen and when they actually hit the button to place the order (which seems to be what happened here). This happened because of promo applicability changes, product price changes, products being de-listed, etc.

But it was relatively easy to prevent problems with this. When the user clicked the button to place the order, we sent along the expected order total, the expected item IDs to purchase, the expected tax, etc. In the backend, one of the last validation step before the order was finalized was just to compare the final order details against those expected values. If there were any differences, the order was rejected with an error message to try again. There were then monitors on these types of errors which would go off if there was a higher-than-normal frequency of these sorts of errors, so we could track down any bugs causing price discrepancies.

I'm kind of surprised that UberEats doesn't seem to have a similar validation step.