If you are worried about the fish, yes buy the copepods and have them overnighted. I bought a bottle and received them yesterday.
However, I have a 144 gallon with a mandarin and lots of fish and at least 200 lbs. of live rock. I am working on a fuge, but I don't have one yet.
As for the purchased copepods, I took the opened bottle and put the whole bottle in the water. With my finger over the top, I moved my hand lower in the tank so I could squeeze it out into the lr, so the copepods could disperse around and away from my filter. BTW, turn off your filter and protein skimmer. Keep the tank or area dimly lit. My mandarin which was out and hunting along with my other plantivores became very active. I could see them eating the more mature copepods. Yes, it was expensive but worth it to make sure my mandarin is healthy until the fuge is together. GOOD LUCK.
And definitely buy some cured lr. I would do a little each week and make a hiding area for your mandarin. Too much at once could cause a whole other issue if it isn't fully cured.