This '/' may be getting in the way of non-.html urls - so there is probably another piece of code to add '/' if it is not product... it may not be htacess issue.
On the other hand there is this in htaccess
Code:
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteRule (^|/)\. - [F]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^((([/_a-z0-9-]+)/)?([_a-z0-9-]+)/)?([_a-z0-9-]+)(\.(htm|html))?$ cart.php?url=$5&last=$4&rest=$3&ext=$7 [NC,L,QSA]
RewriteBase /xcart-devs/xc5
</IfModule>
which works on non-folder and non-directory and with htm or html extensions only. Have you tried any other extension like .hhh to see if it works - if it doesn't then it is definitely this line in htaccess