For fixed fees (e.g. postage), we recommend using a Price field.
For a percentage tax on charges, you can set a value for "Payment tax percentage" under Configure → Payments, or with Custom Pricing Rules.
For a percentage tax on Stripe subscriptions created through Paperform, you can also set a value for "Subscription tax percentage" under Configure → Payments. Note that subscriptions are handled independently of normal charges.
To set a value for "Subscription tax percentage" you need to have connected a Stripe payment account to your form.