How to ship USPS First Class Flats (first class without tracking)

[aliseexpress]

With Endicia, I am able to generate USPS First Class Flats label for items that weight under 1 oz (about $1 per label).

I try to keep it under 5% of all my orders and use it for smallest and cheapest items. However, USPS sometimes (1 in 10) return those to me even that I followed all the rules on size & thickness of envelope. I do use mostly small poly envelopes (same size and thickness, but of course they bend).

Is USPS expecting me to use padded craft paper envelopes that stays rigid and flat? I would like to still ship USPS Frist Class Flats, but without issues from USPS returning those to me from time to time.

[nobody*s_perfect]

To be eligible to be mailed as a flat, your piece must be literally flat (which USPS defines as a variation in thickness no more than 1/4 inch). It must also be within the dimension limits, including no more than 3/4 inch thick. And it must be flexible.  A rigid piece cannot be mailed as a flat. This section of the DMM has entertaining illustrations of the USPS criteria for flexible vs rigid.

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

[aliseexpress]

Why would USPS sometimes return to me envelopes like this (when using USPS First Class Flats). Those are small poly envelopes (no bubble). I do keep all those shipments flat (including items).

Here is example of items that I ship this way (or other small usb adapters):

https://images-na.ssl-images-amazon.com/images/I/61cxNzK1SYL._AC_SL1100_.jpg

Here is poly envelopes that I use:

https://i.ebayimg.com/images/g/zIEAAOSwWCBZlh0T/s-l640.jpg

[southern*sweet*tea]

That adapter isn't flat, that's why it can't be mailed as one.

Flat means...well...FLAT. No lumps, no bumps.

[muttlymob]

Have you taken any of these returned mailpieces to your post office to ask why they are being returned?

[nobody*s_perfect]

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@aliseexpress wrote: ... Here is example of items that I ship this way (or other small usb adapters):

https://images-na.ssl-images-amazon.com/images/I/61cxNzK1SYL._AC_SL1100_.jpg

 ...

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I'm pretty sure that an envelope with that item in it will not meet the USPS definition of "even thickness."