Only buy stamps at the post office.
Years ago, I had some kind of credit card deal where I got $100 back from the CC company if I spent $500. It was about to expire. So I bought $500 of forever stamps at the post office. I still have a hundred or so stamps left, and over the years, it was a good investment as postage rates climbed.
But buying discounted online stamps from ??? Next you will be buying $100 Franklins from China for $60 each.