The 1-to-1 Projects invoicing feature can be used to to bill clients for any work that has been agreed to.
Sending an unexpected invoice is the quickest way to put your client off. Don't invoice the client until you've discussed the project milestones and payment points with them.
Talk to your client about an invoicing schedule for the project up front, when you’re first discussing the project scope. That way, you’ll both have the same expectations about scope and price, as well as when you'll be paid.
Payment for winning designs in contests is handled through the contest handover, DO NOT send an invoice for this purpose.