SSL on a Multisite Checkout Page

Thanks and apologies if this has been asked before. I have a sales site on a multisite, its the root site, the cert is in place and works, almost. My host is saying

“Please set redirect to https:// in the .htaccess file for the secure connection to function properly, and an SSL certificate padlock to be displayed in browsers.”

I don’t want the whole site HTTPS only when you click the checkout button.

How should I accomplish this?

Many Thanks