If you buy the stamps in sufficient quantity you will have them on hand when you need to send a letter.
Try to think before you speak, it keeps you from looking stupid, as you did in your reply to me. Did you actually I think I meant buy one stamp at a time online each time you needed to send a letter? If you did, please go in for a brain check, yours seems to be missing a few parts.
BTW,I wasn't defending the post office, if you read my entire comment you will see where I said that private carriers could do the job much better.