Online Sales Simplicity and Small Business Relief Act of 2019 This bill prohibits states from imposing a sales tax collection duty on a remote seller for any sale that occurred prior to June 21, 2018. A "remote seller" is a person without a physical presence in a state who makes a sale in the state. A state may impose a sales tax collection duty on a remote seller only for a sale that occurs after January 1, 2021. In the case of a small business remote seller (no more than $10 million in gross annual receipts in the United States), a state may not impose a sales tax collection duty on any person other than the purchaser if the sale is made (1) on or after June 21, 2018; and (2) before the date that is 30 days after the states develop and Congress approves an interstate compact, applicable to the state and sale, governing the imposition of tax collection duties on remote sellers.