How PayPal fees work
PayPal's commercial (goods-and-services) fee has two parts: a percentage of the payment plus a small fixed fee per transaction. The standard form is:
fee = ( amount × rate ) + fixed
net received = amount − fee
If instead you know the amount you want to keep and need to work out what to charge, you “gross up” the payment so the fee is covered:
amount to charge = ( target + fixed ) ÷ ( 1 − rate )
Worked example
You receive $100 at a 3.49% + $0.49 rate:
Percentage fee: $100 × 0.0349 = $3.49.
Plus fixed fee: $3.49 + $0.49 = $3.98 total fee.
Net received: $100 − $3.98 = $96.02.
To actually keep $100, charge ($100 + $0.49) ÷ (1 − 0.0349) = $104.12. PayPal then takes its fee and you're left with the full $100.
Why you can't just add the percentage back: adding 3.49% to $100 gives $103.49, but PayPal charges its fee on the higher amount you charged — so you'd come up short. The gross-up formula above accounts for that loop.