JobSlinger doesn't send invoices, but it makes it possible for SASSIE companies to generate them.  IC Pro invoices aren't needed for you to get paid -- SASSIE companies know which shops to pay based on the shop status.  IC Pro invoices are used by mystery shopping companies if the IRS ever challenges your independent contractor status. Invoices are one of the documents that help establish independent contractor status (regular employees don't need them), and by being able to produce an invoice for work that you have performed, the mystery shopping company can demonstrate that you were not a full-time employee. 

Why would I want to give permission for invoicing?

How can I view my invoices?