Amazon and eBay have started to calculate and collect sales tax on behalf of sellers in certain US states. The list of affected states constantly increases. For instance, Arkansas, Indiana, Kentucky were added on the 1st of July 2019.
Due to the new tax rules, a double sales tax collection issue arises. When Channel order from an affected state enters Magento, the tax applies to this order again based on your tax settings.
As of September 2019, M2E Pro Team is working on the issue. It will be resolved in the nearest server update.
Meanwhile, you can use a workaround. Contact our Support Team at email@example.com to get customization that disables tax calculation in Magento orders from the affected states.
Note: The adjustment applies to Magento orders that come via M2E Pro and does not affect your website orders.