Although I used iHerb for many years (and am a member of their VIP program), I found out a while ago that Amazon usually beats them in price - often by a significant amount. This is especially true if you are able to get a supplement under Amazon's Subscribe & Save program, which deducts either 5% or 15% from the price, depending on whether you have at least five Subscribe & Save items in your monthly order. On top of that, if you are in the U.S.and use Amazon Prime (which has many benefits besides shipping), you're eligible for the Amazon Prime Store Card, which gives you an automatic rebate of 5% on all of your Amazon store purchases. When you add up all of these discounts, you can save quite a bit of money.
Normally, I like supporting independent retailers like iHerb, but when I'm on a limited budget and Amazon can save me hundreds of dollars a year, my financial situation makes Amazon the place for me to go for most supplements.