Once you have purchased a service from another user, there are many ways to use it. Remember that a service doesn't need to be consumer in one shot. It really depends on what has been agreed with the other user.
Now to use a service you have purchased, you can that in several manners:
Go to the profile of the user you have purchased a service with. There you simply need to connect with them and start. Depending on the type of service purchased, you can ask the other user to click on the $ sign and enable the calls, video calls or scheduling with you. This is all decided between yourselves.
When in a chat conversation with that user, simply click on call or schedule to get started, as long as the other user has confirmed the payment and/or enable these options for you in their $ screen.