How do I add or remove payment methods to/from a shopping cart

You can add or remove payments methods on a plug by visiting your Plug's sheet. Visit your "Plugs" page, click the 3 dots menu on the right and select the "View" option. Alternatively you can click on the table row. 

Choose the Gateways tab and you will be able to enable and disable the payment gateways you like for your shopping cart. 

Please note that the available payment gateways for each shopping cart depend on the product you are selling.