Does an envelope 8 5/8 x 5 3/4 inches require extra postage
Yes susarocks, I'm afraid you need to pay extra on that but its better to ask USPS directly. You can contact them here : 1-800-USPSSource: http://www.euroffice.co.uk/i/wc7/DL-Envelopes-5-Star-Wallet-Press-Seal-Window-Envelopes-90gsm-White-Pack-1000
Yes, an envelope 8 5/8 x 5 3/4 inches will require an extra postage. Knowing the outline of your mail determines the fee you will pay. Thickness and weight of the envelope may also add to the load of your mailing in that you may incur extra postage.
postage charge is calculated by weight.
No.All extra postage is needed on basis of weight of letter or package.