What exactly do they mean by "Extra Postage Required"?

The V-Day card I bought for my bf says "Extra Postage Required" where the stamp would go on the envelope. But exactly HOW MUCH extra postage do they mean? I have 37 cent and 2 cent stamps (to make the normal 39 cent postage). Should I just put two of each? Although 78 cents sounds like too much, and I don’t know if I can put 4 stamps on there. It’s not an extraordinarily large envelope or anything, it’s only 6.5" X 9.5", and it’s not a real heavy card.

What do you normally do when you need extra postage? Does anyone out there work at a post office and know for sure?

Should I put two 37 cent stamps (even though normal postage is 39) or do I actually need 4 stamps to make two sets of 39s?

This might sound like a dumb question, but I haven’t send a card via snail mail in a loooong time (I use e-cards and e-mail for everything) but I thought it would be nice to send a regular card this time.

Thanks!
Thank you Paul K, I can see YOU didn’t read the whole question… dummy.
#1, I don’t have any way to get to the post office, so I forgot to mention that, and #2, I just wanted to see if I needed double the normal postage or if I COULD use two *37* cent stamps (cuz normal post is 39), because as many of you didn’t read, I don’t HAVE two 39 cent stamps, only 37s and 2s, and I’m pretty sure you can’t put more than 3 stamps on an envelope and I’d need 4 in that case.

So honestly, all I wanted to know was if I could do two 37s or if I needed 39s since that’s what the new postage is. If I could get to a post office, I would have had them weigh it for the exact postage, but I don’t have that option, sorry I didn’t mention that.

If you’re not going to read the details of the question, don’t bother to answer it.