Hi Marc, our eCommerce plugin works fine with the latest version up to 3.8.1 (current). We’ll get the main plugin page updated shortly, thanks for letting us know!
As for the costs, it comes included with your premium license to Apptivo ($10/month/user). There is no separate fee to be paid, just log into your account and you’ll be able to find your keys. Check out this link if you need help finding your API/Access keys:
https://www.apptivo.com/answers/questions/10413/where-can-i-find-my-api-key-and-access-key
