First, a 1099-K is issued by the company that pays you. You do not need to create one in order to pay yourself. So don't worry about the 1099-K.
Second, you are what is called a 'self-employed contractor'. You will enter the total amount that you received (in this case $600) and the total expenses that you had (in this case $300 plus any other expenses that you were not reimbursed for such as mileage). Whatever is left is what you pay taxes on.
@marcarri08
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