Ebay standard envelope mailing

[tackyt65]

Is anyone else having a problem with USPS returning envelopes and requesting additional shipping. My envelope went from Ft. Worth, TX  to Miami, Florida before they decided to return it to me and request additional posting. I complied with the 3/8" requirement and the 3oz requirement. I also complied with the size of the envelope and it was still returned. 

[albertabrightalberta]

Did you have letter (rather than parcel) postage? 

I sent an item to someone with a single stamp. (Didn't need tracking.) Although <.25" thick, it was returned to me with $3.45 postage due. 

Since the item was under an ounce and within the letter rate thickness, I put it in a new envelope (to avoid getting attention from USPS) and resent it. 

It went through the second time. 

Note that if this is for a sale, since there's no tracking, you have no proof of delivery if you get a dishonest buyer who claims non-receipt.