esbit tablets

Yes. Both the PO and TSA consider them flammable (they are designed to do that) and are therefore forbidden.

The fact that they are harder to ignite than the box you're shipping them in doesn't matter; it is a fact lost to them. Almost everything else around them would have to be already burning before the tablets would ignite spontaneously (just try lighting one in the wind).

It is virtually impossible to buy Esbit tablets along the trail, so you might consider putting them in your re-supply boxes and don't tell the PO about them. They don't have the time or capcity to check packages routinely.

When flying to a major city, I'll buy them when I get off the plane and before I go to the trail. I don't put them in my checked luggage (pack) on the off-chance that some other piece of my camping gear might trigger a curious TSA person to actually dig into my pack. I can't risk having it rejected. A box of 12 only adds 6 oz to my pack, and that will do me for about 3 weeks.

Yes, they need to know the contents are flammable.
You can call and ask them the specifics. They may only require marking on the shipping container.

http://thru-hiker.com/articles/flying_with_fuel.php

thought they were fine for mailing

Companies that ship items have two ways of dealing with it – just shipping them and hoping they don't get caught – or they file paperwork and be legal.

UPS allows more stuff to be shipped ground than USPS does.

Lets just say a LOT of people ship stuff they shouldn't. Get caught and playing dumb isn't an excuse.

Had planned to use esbits along the JMT. Aren't they available at resupply places along the trail?

Good point, Sarah. I, too, have purchased esbit tablets thru the mail from sources found at this site.

they are fine to mail via ground within the usa, just can't be airmailed