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Welcome to Rex's Guide to Planted Tanks
The aquarium fert co used to be Greg Watson, and I have not bought dry fertz in a while, but I seem to remember some complaints about shipping costs being VERY high and product showing up in Flat Rate USPS boxes so that they were charging about 200% of the shipping cost or more. It has been some time since I have seen this, but Rex (also along time for me to actually be up to date on) is pretty upfront about his shipping charges and I don't think there is a CRAZY mark up for profits.
You can specify for HIGH GRADE ferts for a little higher price from Rex IIRC, like lab grade or something like that, not sure about the Aq Fert Store.