Paying a bonus doesn't take a lot of time. Can be done in 30 seconds.
You can pay a bonus, and THEN send a message to the freelancer.
If you a send a bonus payment to the freelancer, Upwork automatically sends them a message informing them of the bonus.
A freelancer who sees this message doesn't need to take your word for it. She has the proof that you already paid her that extra money.
Of course that will influence what type of feedback she leaves for the client.