hmm, pretty big fish. Have the buyer pay shipping charges.
If possible, triple or quadruple bag them in bags large enough for the fish (This should be a given but with some people you never know.). I personally like 1 fish per bag.
The way the guy that sells me discus ships them is triple or quadruple bag and bags the fish individually, then bags them up in a larger bag just to hold them together and keep them from tipping. Then they are put in a styro cooler and packing peanuts are put around them. Then the syro cooler is in a cardboard box.
If you can't get a carboard box and styro cooler that fit each other, just go buy some styro sheets and cut it to size for the box, just needs some insulation.
Also shipping this time of year, heat packs are needed.