I had 2 batches of cardinals come through the mail. The first batch I put them in their own tank with soft water (distilled) and a bunch of plants. These were shipped 2 day. 9 out of the 25 survived to make it to me display tank after spending 3 weeks in my quarantine where I slowly acclimated them to my tank water. I fed them flake food but then got smart and fed them white worms. When I out them in my dislay tank they got ich. I treated them in a separate tank and when I out them back in the display, 1 died. So I ended up with 8 out of 25. Not so good, not so bad.
The second batch did much better. This time I got 30, put them in a quarantine tank with soft water for 3 weeks with the same acclimation regime. I fed these guys white worms from the get go. I only lost 6 over the course of 1 week and the rest were great. They are now in my dispaly tank with the rest. The only thing I did different from the first batch was get them shipped over night and feed them white worms from the beginning.
YMMV but sometimes it makes sense to just get them from the LFS at $3-4 a piece. In the end that is about what I spent between the 2 batches I got.