XRP Ledger transaction fees are paid in drops (1 XRP = 1,000,000 drops). Fordefi determines the fee automatically; you cannot select or override it.
Fordefi obtains the fee by simulating each transaction against an XRP Ledger node and using the fee the node returns. This fee reflects current network conditions and may rise during congestion.
When no valid simulated fee is available (for example, for a CheckCash that depends on a preceding TrustSet), Fordefi falls back to a fee of 12 drops.
Flows that submit multiple transactions (for example, a TrustSet followed by a CheckCash) incur a separately determined fee for each transaction.