The latest release of Parrot contains new functionality to manage collective invoicing of your contracts. 

A customer can have multiple contracts set up but still want them to be combined into one invoice. The new feature Collective Invoicing in Parrot makes this possible. Set the field Collective Invoicing on the Contract to yes, and when creating invoices from contracts, all contracts within set rules will be combined into one invoice.

Read more about this and other functions in Parrot here