The requirements of the USPS for envelops are.
Envelopes must be rectangular and made of paper to qualify for letter prices.
Your envelope can be a maximum of 11-1/2" long x 6-1/8" high.
(A standard No. 10 envelope is 9-1/2" long x 4-1/8" high.)
You can fold what you put in your envelope, but it needs to stay flat—no more than 1/4" thick.
For mail to be considered a flat for USPS the mail must be one dimension that is greater than 6-1/8 inches high OR 11-½ inches long OR ¼ inch thick.
Flats with the postal service or USPS refers to magazines, newsletters and large envelopes.
Letters do have to be flat so that the machine can cancel out the stamp.
If you have a letter that is not flat or a lumpy envelope then it will require more postage as it requires hand canceling out the stamp and so a non machinable surcharge will be charged if the letter is not flat.
A non machinable letter through USPS means that the letter is irregular in shape and cannot be sent through a machine to cancel out the stamp.
In the case of non machinable letters the postal employee has to hand cancel out the stamp on the envelope and sort it without the machine basically.
Although the non machinable letter will still go down a conveyor belt and through a machine that reads the address it cannot be canceled out using the machine because it could damage the contents that are in the envelope.
If something is inside an envelope that is bulging the sides of the envelope out then that would be a non machinable letter.
Non machinable mail basically means that the mail cannot go through the machine to be sorted properly to cancel out the stamp so it will need to be manually sorted.
Machinable mail is mail that can go through the sorting machines easily without much human intervention.
Machinable Mail is mail that can go through the sorting machine easily but non machinable mail cannot go through the sorting machines to be read and sorted.
Machinable mail will be able to go through the sorting machine where it can be read by the machine and computer and go through to the proper spot to where it can be delivered to the correct address.
A non machinable piece of mail would be mail such as a letter that has something inside it other than paper such as a pen or pencil, key ring etc.
The item inside the letter will make the envelope be irregular and bulge out which makes it non machinable so then a human has to hand sort it and they will stamp it as non machinable.
Non machinable mail usually requires a bit more postage than a standard letter would require.
A USPS machinable parcel is any package that fits within the dimensions of the USPS sorting machines which means that the parcel can be sent through and sorted through the USPS sorting machines which is a faster way of the parcel being sorted and delivered.
If the parcel is non machinable it means that the parcel is an irregular shape and cannot be sent through the sorting machines.
In that case the parcel requires hand sorting and hand canceling of the postage to get the parcel to where it's headed too.
Non machinable parcels are usually slower to deliver because of the extra steps required by a postal employee to sort it and find out where the package is going too.
So it's best to make sure your package or parcel fits within the dimensions of the USPS sorting machines guidelines so that the machines can sort the parcel and send it on it's way much faster.
When you have a letter that is non machinable then the non machinable letter must be hand cancelled by a postal employee which costs a bit more than a regular letter stamp.
If the letter weighs less than 1 ounce then you'll pay the current cost of a postage stamp and then another 22 cents on top of that for the non machinable surcharge stamp.
So the total cost of mailing a non machinable letter is around 71 to 72 cents which isn't too bad.
It used to only cost me 65 cents a few years ago to mail a non machinable letter but the costs have gone up since then and when I mailed a non machinable letter a few months ago it cost me 71 cents.
So expect at lest 71 cents to mail a non machinable letter depending on the weight.