How much does a letter weigh?

Today I went to the post office to buy some stamps. At home, I'm used to just swinging by the post office and picking up a book of twenty five stamps so that I can have them on hand. I didn't know what to expect here, but I figured it wouldn't be too complicated.

I was not prepared for the third degree from the man behind the counter. His words are in caps because that's what he sounded like. If you don't feel like reading, you can watch this. It's basically the same situation.

Can I buy stamps here? YES YOU CAN BUY STAMPS. IF THERE IS A QUEUE THEN YOU HAVE TO JOIN THE QUEUE BUT THERE IS NO QUEUE RIGHT NOW SO GO AHEAD. Okay... so... I'd like to buy some stamps please.

*At this point he gives me an exasperated look while shrugging his shoulders and holding his hands apart, and I start panicking because I don't know what else he could possibly need to know and I feel like an idiot, so I just kind of stare at him like, "I'm a foreigner. So are you. Can you maybe just tell me what to do?"*

WHAT ARE YOU SENDING? Um, just like a regular letter I guess? Or maybe a postcard or a Christmas card?

HOW MUCH DOES IT WEIGH? (What? How would I know that?) Well it would just be a piece of paper, so... HOW MUCH DOES IT WEIGH! I don't know! It's just a piece of paper! Like a regular letter! HOW MUCH DOES IT WEIGH!! TEN GRAMS? FORTY GRAMS? HOW MUCH?? Ten grams, I guess?

*Let me just pause here to point out that the last time I weighed anything in grams was in Mr. McLaughlin's science class at Reading-Fleming Middle School. That was seventeen years ago. Continue.*

WHERE ARE YOU SENDING? To the US. (That one was a fair question. I should have specified that it was international.) 

HOW MANY? How many? Um, five? (I actually wanted way more than five, but I wasn't prepared to say how many I wanted because I'm used to just buying a pack. I was also getting the vibe that it's not normal to buy a lot of stamps at once here, so I just said five and hoped that I didn't sound stupid.) 

FOUR POUNDS NINETY. For five stamps?? FOUR POUNDS NINETY! Okay.

Good grief.