Renewing subscription ? (MLB At Bat app)

Hello, I am seeking to purchase / renew a one year subscription to the MLB At Bat App, which is about $19.95 in cost.

The iTunes store seems to not wish to allow me to do this just using the store credit in my account. Can subscriptions only be purchased/renewed with a linked card payment set up?

I have a sufficient balance in my account. I do not really wish to link it to a payment card. Redeeming an occasional card purchased retail is fine for me, and helps me keep impulse purchasing down somewhat.

Is it listed somewhere in the terms of service for either the iTunes Store or this particular app that the in-app fees can't be purchased with an iTunes Store balance?

MLB kicks this question right back to being an "iTunes Store question".