USPS Penguin Additional Ounce Stamps - Sheet of 20 Stamps

Can they be used to actually mail?

Can these only be used in addition to a 44 cent stamp?

what is the value of this stamp? i think it used to be 22 or 21 cents and now when i search for additional ounce cost online it says 15 cents!

So if I understand this, rather than pay $4 for $4 of postage at the post office... I can buy $4 worth of postage here for almost $10?