As of January 1st, Amazon is now automatically calculating, collecting, and remitting tax for orders shipped to customers in Washington state. You can read more about their Marketplace Tax Collection and Tax Collecting Service policies in their FAQ.
You do not need to make any changes in Monsoon for the Washington tax collection - this happens automatically on Amazon. When the orders come into Monsoon, you’ll see the taxes included separately from the item prices.
If you’ve set up tax collection on Amazon for other states, you’ll want to make sure you have the tax codes enabled in Monsoon. More info on how to set this up in Monsoon is available here.