Free Delivery fee without a date specified when it will start and when it will end

So'they have this where no delivery fees are charged for all items ordered with no date specified as from when no deliver fees will be charged. On 3 screenshots sent to them they only advise me then that free delivery only started on 18 May 2020. I wish I can send you a screenshot of their free delivery adverti*****t on items already placed and paid for.