Hi all.
Posting this in case it helps other people.
I am on an annual Recreation subscription for my Garmin InReach mini. This should include 40 free messages per month (https://discover.garmin.com/en-AU/inreach/personal/). Reviewing my last few invoices, I discovered that I have been charged for all text messages, even though I hadn't used my free limit of 40. I queried this with Garmin who (to their credit, promptly) acknowledged the invoicing error and credited my account.
So, if you are an InReach user, it's definitely worth paying attention to your invoice for billing errors like this.