Can't make purchases with a mix of gift cards and my bank card

I'm new here and I don't tend to run into issues a whole lot but hopefully I'm in the right place.

Anyhow, I received some gift cards as a present from friends which i redeemed and it totaled up to 30€, and I wanted to cover the difference. The only option that was not greyed out was the card option so I chose that, but it kept failing the purchase. I would appreciate any insight anyone might have on this. 

Thanks in advance :D