you could create a rule specific to each page that should be under SSL:
RewriteCond %{HTTPS} off
RewriteCond %{REQUEST_URI} /checkout.php
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [L]
RewriteCond %{REQUEST_URI} /confirm.php
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [L]
etc....