I've used esbit exclusively for 3 season hiking for many years, including the PCT and the CT. They're great!
CAVEAT: you have to protect them from wind while lighting AND cooking. I light mine with a BIC lighter, holding the flame against a corner of the tablet. Remember that any flame is hottest at the tip.
No, you can not (legally) send Esbit through UPS or USPS. It must go ground, labelled accordingly.
I buy esbit-brand tablets at REI in boxes of 12, or they can be ordered from BPL or other on-line sites. Rarely have I found them along the trail, although they are available at:
PCT/JMT - Mt. Whitney Hostel in Lone Pine (and likely at the Mt Whitney Store itself as it's the same owner) and at Muir Trail Ranch.
CT - Salida Mountain Sports and in Silverton at Outdoor World. There is an REI in downtown Denver that I've never been in, but the Portland REIs all carry esbit so I suspect they will also.
I keep my esbit tablets in their protective packaging inside an OP Sack, which I carry on the outside of my pack. Whenever I ship them, I do the same. YMMV.
Esbit tablets also make great emergency firestarters (12 minute burn time) for those who use some other stove fuel but also carry an emergency firestarter.