OP is talking about credits. If they pay using credits, those will be used to cover fare AND taxes (assuming the credits cover the whole cost).

OP: you can not choose to use part of a credit (you can of course, use a full credit and have a balance to pay with the card if it doesn’t cover the full amount) - you can only choose an amount to use with TravelBank funds. Options with this, if there are multiple credits are to cover as much of the cost in the whole credits you have while leaving one out so you can pay with the card.

alternativelt, any ancillary paid with a card post-purchase should associate the card with the reservation, and add the bag benefit. For example, if you buy the ticket, then add a paid seat assignment (preferred or E+) separately, than that should flag for the free bag. Note it may be cheaper just to forget it (unless it’s an ancillary you want/need anyway), and pay for the bag. But YMMV.