Products, Services, and Fees are the types of line items you can add to an invoice. Each type of line item acts slightly different depending on your selection. 

When editing an existing invoice, you can choose to add a line item only one-time or have it recurring with the subscription plan: 

You will see the recurring symbol on the line items set to recur and remain with the subscription plan.

Widgets & Forms work the same as the invoices and can have either recurring or one-time items added onto a subscription plan.

Screenshot of the upsell widget's typesetting under product settings:

Please let us know if you have any questions. 

Sincerely,
The PayWhirl Team