I hold an ISA with Freetrade. I recently added my debit card to my account to pay for the ISA fee of £3/month.
Taking the ISA fee from the card means it does not eat into my ISA allowance.
Now that you have my card details, why not take the Instant Trade Commission, FX, and Stamp Duty using the card? If users are buying UK companies, the stamp duty on the £20,000 annual ISA limit would be £100.
Does paying by debit card mean Freetrade will incurr processing fee? I guess for Freetrade, it would be a question of finding the balance between the cost Vs benefits in terms convenience for customers.