Bad service from pet heaven

Pet heaven sent R200 off voucher yesterday and advised it expires midnight the 31st July.. I tried using it today 08:10am 31 July 2019 and received an error the voucher is invalid. I emailed them to find out whats happening and they responded that they closed the voucher early because there staff could not handle the amount of orders...Can someone please advise me whose problem is it if you do not have sufficient staff because i work in customer service and surely know it not the customers problem... :-(