Out of stock product voucher

On the 3rd of April 2016 I got a out of stock product voucher from Checkers in Somerset West, voucher nr A***, for 2x 2.5kg Golden Cloud Cake Flour. The lady that issued me the voucher made it valid for on 2 weeks and did not put the advertised price on the voucher. When I wanted to use the voucher at Checkers Vergelegen Plain on the 17th of April 2016 cashier said the voucher expired and there is no price on it. If I had to drive back to Checkers in Somerset West the flour would have cost me more in fuel than what I would have originally saved on the special. So what is the use of a \rain check\" then?"