Refund my extra baggage cost

I bought an extra baggage item but luckily my bags did not exceed the recommended kgs. The issue is that BA is refusing to refund my unused item, saying I should have claimed before check-in...how was I to claim that because, I have to 1st check to know if I exceeded or not.

Please assist as I cannot let my hard earned money to go just like that.....it's unjust and a rip-off especially during this time where we are thru the most financially!!!!!