In Missed.com you can have free and paid appointment types.
When your provider sends you an invitation to a free appointment you will receive an email with an Accept/Decline link. By going through the link you will be asked to accept/decline the appointment. If you accept it, the status will be changed to Confirmed. However, if you want to decline it, you should provide the cancelation reason. Your provider will receive an email with the status of the appointment.
When your provider sends you an invitation to a paid appointment you will receive an email with an Accept/Decline link and if you have not added your card to your account through Settings → Payments → My cards yet then you will receive a second email with Filling out payment details link.
To accept/decline the appointment go through the Accept/Decline link. It takes you to the workspace and asks whether you want to accept it or decline it. If you accept the appointment you will have to add your card details in the next form.
Once you finish the process you will see the Appointment Details with a Confirmed status. Your provider will receive an email about the confirmation of the appointment.
You can also go through the Filling out payment details link which is in your email address to add your card only. The link will take you to the workspace and ask you to enter your card details.