Want to use expiring credit towards music subscription

So I'm trying to purchase a game ($69) via the web store and my $73 credit isn't showing as a payment option. I have to add that the notification email sent regarding my expiring credit explicitly stated that "You can use your promotional balance before
it expires to buy apps, games, music, and more at any of the following stores...", with links to the relevant web stores. So far it seems that I can only use my credit purchasing apps through the windows and windows phone store on my devices.

My xbox is in transit so am unable to make the purchase through it. What other options are available to me? Buying $50 of phone apps is not going to make my day...

It's not possible to purchase any subscriptions with the currency system. So yes, you would need to purchase something else.

Thanks

Would the single membership not be used as a credit towards a family subscription?

100% agree. I don't like using my Lumia 1020 as a music device in the gym, since it's just too big. It looks silly on my arm and bounces around in my pocket. I want a small music player like an ipod, but one that is compatible with my xbox music subscription
and isn't made by apple. There isn't such thing, so I purchased a sony Walkman and a Sony Unlimited Music Pass. Now I have a Sony Unlimited Music Pass and a Xbox Music Pass. I paid for a year subscription, but will likely cancel the Xbox Music Pass when the
subscription expires in March 2014.

Depending on how frequently you used the song credits, the new Xbox Music Pass might actually be a better fit for you. Essentially, you pay $5 for $10 worth of music. If you don't actually use all your credits, that's a waste because the credits expire.
If you spent an extra 5 a month in Microsoft Points, you wouldn't be wasting it.