What does “abusing USPS Flats” mean?

[kath.layn]

Just seeking clarification.  In a recent post the OP talked about sellers abusing the USPS flats.  What does that mean?

[stainlessenginecovers · Accepted solution]

usps flats are basic envelopes, under 1 oz, that ship with a stamp. Sellers shipping 'coins' or small jewelry this way should be shipped 1st Class. They should also ship with tracking, which comes with 1st Class as buyers can simply state "didn't receive" and force a refund. 

[kath.layn]

Ah.  That makes sense.  Thank you.

[nobody*s_perfect]

That explanation might make sense to you but it is completely incorrect.   

USPS has 3 categories of First Class mail: letters, flats (AKA large envelopes) and packages.  The weight limit for flats is 13 ounces.  Other limits on flats include thickness no more than 3/4 inch, even thickness, and they must be flexible.   You can see details about these criteria here:

https://pe.usps.com/text/dmm300/201.htm#ep1096318