How PayPal Fees Are Calculated
PayPal charges most US business payments a percentage of the amount plus a fixed fee. For a goods and services payment through PayPal Checkout the rate is 3.49% + $0.49, so on a $100 sale PayPal keeps $3.49 + $0.49 = $3.98 and you receive $96.02. The fee is rounded to the cent. If the payment comes from a buyer outside the US, PayPal adds an international fee of 1.50%, which makes the same $100 cost $5.48.
Because the fixed fee doesn't shrink with the payment, small transactions pay a much higher effective rate: $0.49 on a $5 payment is almost 10% before the percentage is added. The calculator shows the effective rate so you can see this.