You can use vCita to give your clients the option to schedule time or a service right from any email you send.
You can send them an email inviting them to schedule time with you, or add a link at the signature of every email you send - saving back and forth communication regarding availability, timezone, phone numbers and payments.
Simply click here, and copy the "set a meeting" link, and paste it in your email.
You can also choose a link to schedule a specific service.
You may also send an email campaign with a scheduling option in it.
You can add the scheduling link from above into any third part email marketing such as AWeber, MailChimp, VerticalResponse and others.