There’s nothing wrong with that, though it really should be more obvious. The install directions mention the API:
Update your settings in the Pingler Options Page under the settings menu a. Get your API key from your PIngler account and insert it here. b. All additional options will appear once you have entered your API key.
But yes, the price is not mentioned, and that’s really just a documentation issue.
There’s no rule about plugins being free.
The plugin IS free. Registration is required and you get five free pings daily. If you want more than five pings a day, you may sign up to a premium account.
Registration and the basic use of the plugin are completely free.
Even if it’s not, that’s okay, just needs to be explained a little better on the readme is all 🙂 The FAQ at least should say that you need the API, and that the first five pings/day are free, etc etc.
Think of it like you’re writing for a rank newbie, and try to anticipate what someone who knows nothing might ask or want to know 🙂
