The United States Postal Service (USPS) bases pricing of mail on a combination of size, weight, shape, destination, item valuation, and class of service selected. This is true whether the item is shipped from one United States city to another or to an international location. All of the postage costs provided in this answer are from the USPS Calculation web page.

http://postcalc.usps.com/

Rigid envelopes or envelopes exceeding the maximum thickness of 1/4 inch cost more to mail. Below are some examples of the postage required to send a 6 inches wide by 9 inches long padded envelope from one United States location to another using the First-Class Mail Parcel class of service. With a single USPS Forever stamp currently worth $0.49, just multiple the number of stamps required to match or exceed the postage price listed for the weight being sent. For the 6.15 ounce padded envelope mentioned in the question postage would be for seven ounces which is $3.04, and takes seven Forever stamps to mail.

First-Class Mail Parcel

Customer supplied small padded envelope or over 1/4 inch thick envelope

one ounce - $2.32 (takes five Forever stamps)

two ounces - $2.32 (takes five Forever stamps)

three ounces - $2.32 (takes five Forever stamps)

four ounces - $2.50 (takes six Forever stamps)

five ounces - $2.68 (takes six Forever stamps)

six ounces - $2.86 (takes six Forever stamps)

seven ounces - $3.04 (takes seven Forever stamps)

eight ounces - $3.22 (takes seven Forever stamps)

nine ounces - $3.40 (takes seven Forever stamps)

For faster delivery, there is the option of using a free USPS supplied Priority Mail Flat Rate padded envelope. The flat rate postage price is a fixed amount whether you are sending 6.15 ounces or four pounds.

Priority Mail Padded Flat Rate Envelope - $6.10

USPS Supplied Envelope - 12-1/2 inches long by 9-1/2 inches wide

Consider taking the packed, sealed, and addressed envelope to a local Post Office and have the Postal clerk weigh the envelope so you only pay for the exact postage.

go to the post office and weight it 1 stamp for each 6 ounces so you need 7

The USPS has a postage calculator normally for those with a postage printing account. I believe you get 1 oz. with a first class stamp, so maybe 7 stamps, it might not qualify as first class mail though

https://www.usps.com/ship/.

5 forever stamps.

that will be the small package rate, so probably more than $3.00 in postage