1) If most Chinese people make as much as and have a comfortable life as American **on average**, then 誠信 (or rather lack of 誠信) would not be a problem. This comes back to the phrase we are all too familar: 經濟基礎決定上層建築. This is the bottomline.
2) We do see some oversea Chinese who actually "made it" meaning already well into Amercian "Middle Class" if not higher, yet still behave and act lack of 誠信. Similarily we see and read some Chinese "大款" behave and act like jerks. How come people still do that when money (經濟基礎) is no longer an issue?
Rome was not built in one day or rather 三代出貴族. That's why in my opinion.
Do not confuse two different concepts: credit 信用 is not the same as 誠信。Credit kind of force people to do things according to rules of credit. 誠信is more like truthfulness, not telling lies. Americans would tell lies as easily, if not more, as any other people. You can tell this easily from the politics if you are not close enough to Americans who tell lies.
As for safety of mail, I lost my phone card before in the mail which was used a few hundred dollars. I lost my credit card in the mail before and was used too. Scams from the phone and email etc are everywhere.
Beware of the things that may happen to you from these 『誠信的』Americans. If you let go of your guard, it may happen to you in a great way one day. Do not be too naive.
You presumptions of Americans being more truthful does not stand itself. Rich people do not make more honest people either. They make bigger lies and cover them up with a professional looking. Just look at the politicians. If it happens among politicians, it happens everywhere in America in each and every circle. Do not defame ourselves. Many foreigners think Chinese people are more honest too.