Out of stock and bad customer service

Three weeks back I went to post office to buy an envelope and the lady working there said they are out of stock. Today I went back to buy an envelope and they said they don't have envelopes. And the lady who was assisting me has a lack of customer service. I asked how long will it take for a post to be delivered in Germiston and she wasn't even listening to me, she was busy on a computer.