Sainsburys Students money saving booklet

Picked up at the cash tills a Student money saving booklet which entitles you to save £15.00 of certain products , used it yesterday and all vouchers worked even though we only had a couple of the products in our trolley. Might not always work might depend on the cashier on wether they check them off,

Please correct any details if i have them wrong as im new to this,

Thanks