Payment question.

Firstly, I do wish to join the guild, but thats another matter which links to this.

I have 30 days, which for some reason bugged? down to 25 days upon activation. Anyway, Since it wouldn't accept my debit card, I have to use paypoint. If anyone uses this method to pay for the subscription, please do tell how it works? Do I simply input the code when it asks for it? How comes I see nowhere to put it in? What if I get a 10 Pound top-up, does it use it all?

Thanks in advance,
Taylan.