Вам надо ваши правила редиректа выше поднять, сразу после RewriteEngine On, переставить их местами и добавить флаг L. Попробуйте привести .htaccess к такому виду:
# 1.To use URL Alias you need to be running apache with mod_rewrite enabled.
# 2. In your opencart directory rename htaccess.txt to .htaccess.
# For any support issues please visit: http://www.opencart.com
Options +FollowSymlinks
# Prevent Directoy listing
Options -Indexes
# Prevent Direct Access to files
<FilesMatch "(?i)((\.tpl|\.ini|\.log|(?<!robots)\.txt))">
Order deny,allow
Deny from all
</FilesMatch>
# SEO URL Settings
RewriteEngine On
RewriteCond %{HTTP_HOST} ^www\.(.+)$ [NC]
RewriteRule ^ http://%1%{REQUEST_URI} [L,R=301]
RewriteCond %{HTTPS} off
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
# If your opencart installation does not run on the main web folder make sure you folder it does run in ie. / becomes /shop/
RewriteBase /
#RewriteRule ^sitemap.xml$ index.php?route=feed/google_sitemap [L]
RewriteRule ^googlebase.xml$ index.php?route=feed/google_base [L]
RewriteRule ^system/download/(.*) index.php?route=error/not_found [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !.*\.(ico|gif|jpg|jpeg|png|js|css)
RewriteRule ^([^?]*) index.php?_route_=$1 [L,QSA]
### Additional Settings that may need to be enabled for some servers
### Uncomment the commands by removing the # sign in front of it.
### If you get an "Internal Server Error 500" after enabling any of the following settings, restore the # as this means your host doesn't allow that.
# 1. If your cart only allows you to add one item at a time, it is possible register_globals is on. This may work to disable it:
# php_flag register_globals off
# 2. If your cart has magic quotes enabled, This may work to disable it:
# php_flag magic_quotes_gpc Off
# 3. Set max upload file size. Most hosts will limit this and not allow it to be overridden but you can try
# php_value upload_max_filesize 999M
# 4. set max post size. uncomment this line if you have a lot of product options or are getting errors where forms are not saving all fields
# php_value post_max_size 999M
# 5. set max time script can take. uncomment this line if you have a lot of product options or are getting errors where forms are not saving all fields
# php_value max_execution_time 200
# 6. set max time for input to be recieved. Uncomment this line if you have a lot of product options or are getting errors where forms are not saving all fields
# php_value max_input_time 200
# 7. disable open_basedir limitations
# php_admin_value open_basedir none
