Mandarins are quite possibly one of the most beautiful fish in the world in my opinion, but given the knowledge they can release mucus due to stress into a bag and die of suffication even on the way home from a local store, I would never order one online. Just my opinion though.
The other reason to ALWAYS buy one local is so you can see if these fish, notorious for starving, have a pinched in flank. If so, they are NOT eating. My suggestion is to buy a local one, see that he looks healthy and well fed, and DEMAND to see the store feed him if they believe he will eat anything other than live pods.
If he is fat and healthy, and you SEE him eat brine, then OK, but most won't touch brine in my experience.
On a side strange side note, I have kept 6 to date, and they actually did much better when I was new to the hobby...why? Because my tanks had a ton of algae, and the algae created tons of pods, but my tank was years old, and had over 200 pounds of rock. The water sucked and had nitrates enough to give a yellow tang those red lips.... they still lived for years and ate pods day and night.
They did less well after I was in the hobby awile and had more algae free sterile tanks because the pod populations were more scarce due to lack of food (algae). Just my quick thoughts on the matter for what it's worth.