You're to do two things for this client 


Change the Selar payment link on this page to stripe 

https://www.sustainablydrgaz.com

Stripe payment for $15
 https://buy.stripe.com/28EaEW49q4koba86Wa6Vq00

Then, change the WhatsApp thank you page link there to this


https://chat.whatsapp.com/DWukiBEY8ut0aTfw7LXC79?mode=wwt

Here is the thank you page 

https://www.sustainablydrgaz.com/thank-you

Ensure you test that everything is well set before you replied as done