Ducksell Plugins:
2Checkout intergration
(https://github.com/interactive32/TwoCheckout-DuckSell-Plugin)
PayPal intergration (https://github.com/interactive32/PayPal-DuckSell-Plugin)
BitPay integration
API plugin
Customer support
Customer Files
Blue customer area
Free product
Mailchimp intergration
Download info
Custom invoice
Product emails

Sign in to your DuckSell application as admin.
Navigate to 'Manage' under plugins on left side menu.
Click on 'Add New' button.
Using 'Browse' button find your plugin zip archive and click on 'Submit' to upload it.
After upload has been completed you will be able to see new item on plugins section where you can go to configure your new plugin.
Note: If you are unable to upload plugin you can manually unpack plugin archive and upload the content to this directory: your_public_path/shop/inc/app/Plugins/