When the DIY Tax Exempt Checkout is installed, why are some customers still charged sales tax?
Shopify tracks the tax-exempt status of visitors by their email address. When the DIY Tax Exempt app is installed, the tax-exempt widget is presented on the checkout page.
The website visitor is prompted to provide their email address. When the visitor declares tax-exempt status, the email address entered is flagged as tax-exempt in Shopify's database.
When the customer enters the checkout process, to preserve their tax-exempt declaration, the same email address must be used.
If the visitor provides a different email address on the Customer Information page, then Shopify will not honor the tax-exempt declaration declared on the Cart page.
To ensure that the tax-exempt declaration is honored, the same email address entered in the DIY Tax Exempt Checkout widget on the Cart page must be used on the Shopify Customer Information page.