Either Imhotep or Imhotep's Mum seems to have got a little confused somewhere along the line.
Cooperative Society's used to give 'dividends', based upon the total spend during a specified period. Many Societies switched to using dividend stamps, which generally paid a far lower percentage because of the administrative costs. However they were completely separate from Green Shield stamps, which were issued by garages and convenience stores (among others). Their catalogue offered apparently wonderful gifts which could be obtained just by collecting the stamps. The problem was that you needed to spend a lot of money to get the necessary number of stamps.
For example, back in the 1960s, spending £32 (which was more than 5 times my first basic weekly wage in 1968) could get you a brush and comb set!!!
http://www.retrowow.c...en_Shield_Stamps.html