XSX Pre-Order payment

I managed to get a pre-order in on the Microsoft Store this morning with my card as the payment option. I have since redeemed some gift cards and would like to make sure they are used as part payment but can't see any options to do that. Is it an automatic
thing or how do I go about sorting it out? If I need to ring the MS Store support, I may give it a few days to quieten down. I also have a birthday coming up so gift cards may be the thing to ask for if they can just go straight towards my order.

Thanks

:)