Recurring donations can only be made by credit/debit card and they can be monthly, 3-monthly, 6-monthly or yearly.
The minimum amount you can pay is $15.
The idea is that you don't have to worry about your board running out of credit - for example, you can set your payment up (say $15) for 6 monthly and the payment will be made automatically without you having to remember to do it each time.
For Fixed Plan boards this is foolproof as it's always $2.50 per month.
For Variable plan, you'd have to check your bandwidth screen occasionally to make sure you weren't using more than about 425 MB monthly, at which point you have to start paying extra.