Payment processors usually combine a percentage fee with a fixed fee. The percentage portion scales with the order amount, while the fixed fee is charged per transaction.
That fixed fee is why low-priced products can feel expensive to process. A $0.30 fixed fee is only 0.3% of a $100 order, but it is 6% of a $5 order before the percentage fee is even added.
Use the PayPal and Stripe calculators to estimate net amount after payment fees. Then use a profit margin calculator to include product cost, shipping, platform fees, and other expenses.