Change your configuration to this (add a slash):
RewriteCond %{HTTP_HOST} ^example.com$ [NC]
RewriteRule (.*) http://www.example.com/$1 [R=301,L]
Or the solution outlined below (proposed by @absiddiqueLive) will work for any domain:
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]
If you need to support http and https and preserve the protocol choice try the following:
RewriteRule ^login\$ https://www.%{HTTP_HOST}/login [R=301,L]
Where you replace login with checkout.php or whatever URL you need to support HTTPS on.
I’d argue this is a bad idea though. For the reasoning please read this answer.