Integration ow Wordpress site with Paypal

****Update - Additional Requirement 3 added ****

I'm developing a Wordpress application that connects to Paypal in two places and a connection to Google Calendar:

1. https://buteykoplus.com/reservations/
Appointment booking system with prepayment (Easy Appointments plugin) which routes directly to Paypal. It gets as far as allowing credit card information to be input but the card is always refused (Paypal - "We're sorry - things don't appear to be working at the moment"

2. https://buteykoplus.com/registration-for-the-breathing-course/ goes through Sendowl, which is linked to Paypal. Similar situation - get as far as entering the CC information but get a similar error message.
There is something I'm not setting up correctly in Paypal, and I need help with this. I'd like to set up an appointment to work with me remotely with Teamviewer.

3. Integrate the appointment booking system (Easy Appointments plugin) with Google Calendar

My goal, apart from fixing these immediate issues is to develop a relationship I can rely on in the future to fix any issues that might come up

Thanks / Derm