Card networks charge processing fees on every transaction, and we pass a portion of that through rather than hide it in the exchange rate — it's listed by amount on the fees page.
Bank transfers (ACH) carry no processing fee from us because they don't go through a card network, which is why we recommend them for larger or recurring transfers.
Debit cards are typically cheaper than credit cards to process, so linking a debit card usually lowers your fee compared to credit.