alanh
Re https, what does "doesn't work" mean ?
You can use the default SME server self signed certificate, but your ecommerce team would have to add an exception &/or include your self signed certificate in their browsers trusted certificate store.
If they are the only people accessing the site via https, then it is a pretty easy fix for them to do.
If you have an ecommerce site hosted on the SME server, then I would have thought you would want a good quality commercial certificate installed on the server anyway, for all users to see a trusted ecommerce site ???
Of course letsencrypt will provide that.