how do I add $75 to a project

If you are an employer you will need to ask the freelancer to issue an invoice for $75 so you can make the payment, currently there is no way to add "additional" to Safepay or Invoice if you want to include payment for additional time/effort or as a bonus.

If you are a freelancer, just issue an invoice. Just make sure that the discussion to do the extended work and additional payment were agreed to and in writing (workroom chat will do).