Payment Plugin: Paddle

1. Create account in https://www.paddle.com
2. Provide vendor ID on this configuration page.
3. Provide Public Key from account settings
4. Create products in Padle.com:
   1. Select type of product
   2. On question: "Does your product use the Paddle SDK?", answer "NO"
   3. On question: "How will your product be delivered?", select Webhook
   4. Set prices
   5. Upload some necessary stuff (even empty file or image)
   6. Set Webhook url to: https://example.com/amember/payment/paddle/ipn
      1. Request Method: POST
      2. Choose: Checkouts produce a single webhook request with a quantity attribute
   7. Release
5. In the main products grid, click on "Checkout link" of every product, and specify "Product ID" and "Checkout Secret Key" field for every product in amember product configuration page.
6. Product need appear in top grid, not "Under Development"

Plugin supports upgrades with several limitations: Paddle do not allow to upgrade to a product with less period. Only pro-rated upgrades are supported, paddle in this case will calculate amount itself.