Most likely that you don't have enough funds from that card to fully cover the charge for that app. This happened to me too when I redeemed my $15 gift card in addition to about a dollar and a quarter remaining in my funds to purchase a 14.99 app. The download was interrupted by a billing prompt like you are experiencing and so then I settled a cheaper app which downloaded promptly without a hitch. So make sure you have enough funds and it should download without the necessity of accredit card. Hope this helps.