Is it possible to open the product on the way to the checkout?

There is no direct prohibition to consume food products until they are paid for in the Law "On the Protection of Consumer Rights" (https://zakon.rada.gov.ua/laws/show/1023-12#Text). However, until the customer pays for the product, it is the property of the supermarket. Employees can only make a remark and ask to pay for it at the cash register. If the buyer refuses to pay, they can call the police.