Log in to your Seller Assistant App account

On the left menu select “Extension Settings” in the settings.

Select Amazon Marketplace
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Sales Tax Configuration
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Retailers collect sales tax on orders shipped to addresses in states where a sales tax is applicable. The amount of tax charged varies depending on the state and local tax rates in the buyer's location. At the time of purchase, the sales tax charged will be displayed at checkout.

Fill in the interest rate in the input field.
You need to choose sales tax due to certain state where you buy products.

The settings will be applied automatically.

These settings will be also displayed in the Seller Assistant App extension on the Amazon product page.
Click into COG, fill in the price and Sales Tax will appear.
It applies automatically in the plugin, in the COG field, if the "Apply sales tax" checkbox is selected in the account settings.

Also, you can change Sales Tax manually if you want to. In this case it will apply only to a certain product.
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You can read more about How to Configure Expenses (FBA / FBM Logistics)