What's "self-interested" mean?

Is the expression "be here long" commonly used in announcements?

Yes, "won't be here long" is commonly used in advertisements to say that a product will not be available for a long time, so consumers should hurry to get it while it's available. This type of phrase is used as a marketing tactic. Ex: Pumpkin lattes won't be here long, so come in and buy one today!

In what situation is 'affirmative' used?

'Affirmative' in this context, is used as an exclamation to express agreement, similar to 'got it', 'understood', 'roger that', The term is usually used in military communication for clarification, one wouldn't used it in every day conversation. Ex: A: You've got permission to strike. B: Affirmative.

What does "take your time" mean?

"Take your time" is an expression that means there's no need to rush or hurry. Ex: A: One moment, sir. I will give you your change. B: Take your time. Ex: There's no need to return the textbook to me right away. Take your time.

Why did she use "again" there?

She says "again" because "thanks again" is what native English speakers say when they have already thanked someone for something, and they are thanking them an additional time. Therefore, she already thanked the other girl previously, so she says "thanks again" because she is thanking her an additional time.

Do people really say that "redheads are very sensitive to pain"?

Haha! No, I don't think so! I think Conan is making a joke about how he is in pain from getting a weave because he has red hair. He's trying to make an excuse as to why he's complaining so much.