You can deduct Processing fees as an expense for your business. Based on the tax form, it will be in different places, but it is counted as Business expense.
This is totally on your discretion, if you wish to consider these fees in the price forming or not.
Usually you should be able to find total amount of fees you paid on the statements from the platform (Square, PayPal etc.). Or calculate, based on your Gross sales and money deposited to your account. Difference will be your fees.
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