Why does PayPal charge 4%?

Could you elaborate on the reasoning behind PayPal's decision to charge a 4% fee for certain transactions? Is this fee standard across the industry, or does it vary based on specific factors? Does PayPal offer any discounts or fee reductions for high-volume or frequent users? Is there a way for customers to avoid or reduce this fee, such as using a different payment method or meeting certain criteria? Lastly, how does PayPal justify this fee, and how does it compare to the fees charged by other payment processors in the market?