How big are the fish you're shipping, and how are you shipping them? At this time of year with the weather we're having, overnight is the only option if you want the fish to show up alive and in good condition. It's far too cold to risk 2-day or Priority mail IMO.
I get all of my shipping supplies from Jehm Co,
www.jehmco.com. They have 25-hour heat packs $0.80 each. I also don't trust the breather bags with cichlids because those hard dorsal and anal fin rays will puncture right through a thin bag. I triple-bag almost all the cichlids I ship, even the baby ones. Just as an example, I bagged up some 2" severums a while back and when I dropped one into a bag, its anal fin hard ray caught on the inside of the bag and punctured through a 4 mil bag. Those are thick bags!
I use 1/3 water to 2/3 oxygen (welding-grade works), a Bag Buddy tablet to calm the fish and release oxygen, then rubber band the tops really tight. I only put one fish per bag unless they're really tiny. I put a heat pack on the bottom of the styro, then a layer of bubble wrap, then I stack the bags in the box and loosely pack bubble wrap in between the bags to keep them from rolling around and hold them upright. I shove another heat pack or two down in the bubble wrap between the bags, then tape a last heat pack to the underside of the styro box lid. Put the lid on, drop it into a cardboard box that fits tightly, and tape the hell out of the box to keep it closed well.
Most of my boxes end up being 10 - 15 pounds which cost usually between $75 - 130 to ship FedEx Express (overnight).