Where do you enter payment processing fees in Turbotax?

Enter them in the Other miscellaneous expenses section under a category of your choosing ... bank fees or processing fees would be appropriate.    Use this section for any expense that doesn't fix in the other 17 categories. 

Be careful ... sometimes the fees have already been deducted from the gross sales and the 1099 only reports the net after fees.   If so you cannot deduct them again.  

these aren't fees for item sales, it's for accepting payments for services using PayPal and Stripe.

Same answer.  Enter it as a misc expense.