Morestore — poor attend /rajouri garden /regular visit

Address:

New Delhi, Delhi, 110026

I visit more store in rajouri almost every day.
They have introduce the offer in which buy any 5 biscuits get 15%off.
I did same and purchase the biscuits 9 in number but got discount only on 5.
Then after billing got to know the offer is for 5/10/15.
But my question is that how would u expect it customer to be aware of it.
If the person on billing counter is busy in his own. World and payingno attention to customer