I need to add a 'date and time' option on an existing WordPress site

General information for the website: The site sells 'life coaching' via Skype. There is a payment process on the site already - but it is a simple 'transaction' payment process. We want to give customers the ability to pay - and BOOK a TIME slot and day of the week. So for example, book on a Tuesday at 7pm-8pm for their service.
Kind of development: Customization of existing website
Num. of web pages/modules: 1
Description of every page/module: Want to add into the existing payment process - the choice to book a time and date for the customer. The site is www.deborahwales.com
Description of requirements/features: The integration should be a plug-in or something tweaked to the existing e-commerce plug-in (LearnPress).
Extra notes: The site is www.deborahwales.com