If you only want the uninterrupted service for one month you can make the standard repeating donation through PayPal and then cancel it before the next payment.
The website says you can donate $100. for a year but nothing about one time donations.
It also says you can donate $60 for a year which is the way I did it. That said, as already noted, you set up a $5 monthly donation and then you can immediately cancel and the sub will continue until the end of the month long sub and then simply stop and you go back to sub nags....
Limited time offer
