Provides an all-in-one registration point for subscribers to your blog. Uses Paypal API and recurring billing.
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Your server needs something like php5-curl (if on Debian) to run this plugin and talk to Paypal. If you don't have shell access ask your host to fix it up for you. Else just ask me!
Read the error. Try checking your API credentials, try running without them, and try checking your currency code. Also ensure that you're not doing something impossible like setting 30 months in a year. 'Did you try turning it on and off again?' - obligatory IT Crowd ref.