Not amused

There was a special on skechers shoes. I order. Buy 3 for promotional free delivery. Then order split. Then email received stating supplier has no stock of 1 pair shoes. I am mailed I will be credited. Today credit comes through but for 20 rand less than I paid for the shoes? Yes it's merely 20 rand but why am I penalised because your supplier cannot deliver? Isn't that unfair?