Unable to take payment

I’m trying to purchase this add on: Shortcode Creator. Whether I click Single Site or 2-5 Sites option, after a few seconds I get a popup which says:

Unable to take payment
The merchant has not setup their account correctly so it is not possible to charge you for the product you are attempting to purchase. We recommend you close this window and inform the merchant of this problem.

Worse, there seems to be no way to contact the company directly without premium support. And you can’t get premium support if you don’t buy their product, so they’ve kind of cut themselves off from any and every new customer.