In order to force https:// in payment and other secure pages even if your customer try to access http:// or they forget. You can use .htaccess file and simple code.

Example –
I had created a page http://www.phpmind.com/blog/
I want my visitor to access https://www.phpmind.com/blog/
Here is code which will force them to redirect to https://

RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}