Affinity billing works differently for the bank feeds. Rather than a £15 + VAT annual fee, it’s charged daily, like the rest of the fees. This can be added on a per profile basis. So if you have 5 connected profiles, and wish to enable it for 2 of them, you would need to add it to those 2, and the fees will be calculated based on just those 2.
There’s more information here:
I hope that makes sense, but please do let me know if you have any further questions