Besides the hourly contracts weekly payments or milestones payments, you can issue a bonus or an extra payment to your freelancer.
To do that, log in to your account and go to the My Jobs - Active Jobs page:
On the Active Jobs page, you'll see the list of all the freelancers with an active contract with you. Click on the three little vertical dots on the right of the freelancer, where you can find all the options related to the job, including "Give bonus/reimbursement":
Clicking there, a window will open, where you can fill in all the information. You can select "Bonus", "Expense Reimbursement" or "Miscellaneous Payment" as the reason for the extra payment:
You also find all your jobs on your dashboard as soon as you sign in, where you can also access this option next to each freelancer you hired.