I bought an inreach mini but the card authorisation keeps failing. Tried several cards. What’s my options here? Who do I contact?

Open a support ticket. Use one of the links at the bottom of this page. I'd suggest call or chat. Judging from forum feedback, email is a black hole.

https://support.garmin.com/en-US/?contactOverride=true&productID=592606&tab=topics

The above link is for the Mini itself.  The following link for subscription billing might apply. Or not. I'd try the Mini link first. They can redirect you if it's not the right pigeon hole.

https://support.garmin.com/en-US/?topicTag=region_subscriptionbilling

When you cross-post repeatedly, it's really difficult to help you out. The above are Garmin US support links. I do not have any information on Europe, nor do I know where you are located. Go ahead and try the US links. They probably cannot help, but they should be able to give you correct links.