I sometimes get things from iHerb in better condition than on Amazon (especially if from a third-party seller: I've sometimes gotten something with a broken lid or something like that, through Amazon).
from iHerb if I order a certain amount, the shipping is free. From Amazon if I want several things and am particular about the brand or ingredients, it tends to come from several different Marketplace sellers, so it's harder to get free shipping deals.
Nonetheless, if I have one or two things and it's a better price or I waited too long to order and it's something they stock in Prime, I will order from Amazon. There are also other places, but I don't tend to have a lot of energy for shopping around.
You have to place an order at iHerb to get your own discount code to share. Then if someone uses it, you will get a credit of an amount from their marketing budget (not related to the amount someone buys).
A discount code can be used only on the first order and only by someone who does not share an address with a current iHerb customer.
(my iHerb code is WNR209, but don't feel obligated to use it).