What does extra postage required mean on an envelope?

Customers must affix additional postage when mailing letters weighing in excess of 1 ounce and/or letters subject to the nonmachinable surcharge or mailpieces subject to another rate of postage (e.g., large envelopes or packages).Click to see full answer. Keeping this in consideration, when an envelope says extra postage required How many stamps?For any domestic mail piece that weighs more than one ounce, you must include additional postage along with your Forever Stamp to ensure USPS delivery. Also, you may purchase higher denomination postage stamps for heavier postage at the Post Office™. Avoid placing two Forever Stamps on a mail piece for heavier mail.Furthermore, can I use 2 Forever stamps for extra postage? You can use more than one Forever Stamp if you need to send a package or a letter that weighs more than an ounce. Each stamp is worth the current first-class rate (not what you paid for them). So if you paid $0.49 and the rate rises to $0.50, you can put two Forever Stamps on a package to get $1.00 worth of postage. Just so, how do I know if my envelope needs extra postage? To be eligible for the standard letter rate and envelope cannot be more than 6.125 inches in height by 11.5 inches in length and have a thickness greater than 0.25 inches. Anything that exceeds these dimensions will incur additional postage.How many stamps do I need for a thick envelope?Generally speaking, you can mail 4-5 pages of regular paper, plus an envelope, for the regular first class (“one stamp”) rate. Heavier paper, rigid or oversized envelopes will cost more. You can get about 10 pages of standard page weight, plus an envelope, for the two ounce rate.