I have the same concerns as the original poster, I understand its not an eShop issue and its not a security issue.
But it IS a customer experience issue. I got this same warning when I purchased the eShop UPS rates plugin today and it made me feel uneasy.
It’s just not ideal to have a warning pop-up that falsely informs the customer they’re financial information may be easy stolen.
You mentioned this is a browser warning when going HTTPS > HTTP…
So if I do use SSL the error will be gone right? or will that break the eshop plugin?
And if it does break the plugin can I use a subdomain somehow to separate eshop and the SSL thank you page.
I’m fairly new to using SSL so any ideas you may have would be much appreciated.