Wow, sorry for the losses. You didn't do anything wrong, it looks like you had some sick tetras from the get-go. Have you contacted the seller about this? Also, I would not be very happy about receiving some neons with the cardinals, especially given the difference in price.
Glad to hear that your
SAE is improving. They are tough fish and I doubt you'll have any problems with yours. Amanos are pretty bulletproof and they ship great, so you shouldn't have any trouble there.
If you decide to treat your fish with any meds, I would strongly suggest isolating the sick fish in a quarantine tank and treating that tank rather than your display. It depends on the med, but many meds are very toxic to inverts and can kill them quickly.