customer has to endure extra costs

On the 10th September, I collected a post from Southdale branch which I tried to repost back but I was told that they had ran out of speed service bags. I then returned to the branch where my courier service was supposed to be returned to on the 11th of September, and I was told that they also do not have bags. I spoke to the manager @ Mondeor branch and he told me that I had to pay an additional extra cost cause they didn't have the bags. I then queried the fact of why I should pay an extra cost if the company did not budget for expected sales and have enough stock. I was told \it's not his problem that post office doesn't have those bags in stock