How do I contact Klaviyo about billing when I've cancelled my plan?

I’m trying to contact Klaviyo to get them to refund me my bill after I cancelled but it’s absolutely impossible.

Because my plan is cancelled, I can’t use the live chat or email function. I’ve raised a ticket which they’ve replied to but I cannot reply to them as the email bounces back because apparently it’s spam. I don’t want to spend all day trying to find out how to contact Klaviyo, I’m trying to run a business!! 

All I want is a refund for my cancelled plan. The plan was cancelled on the day the money was taken!