[cflsystems]

The line

RewriteRule ^((([/_a-z0-9-]+)/)?([_a-z0-9-]+)/)?([_a-z0-9-]+)(\.(htm|html))?$ cart.php?url=$5&last=$4&rest

looks for only htm and html to rewrite url. You can try adding hhh to see if it will work - if it does it is definitely this rul

RewriteRule ^((([/_a-z0-9-]+)/)?([_a-z0-9-]+)/)?([_a-z0-9-]+)(\.(hhh|htm|html))?$ cart.php?url=$5&last=$4&rest

You can also try

RewriteRule ^((([/_a-z0-9-]+)/)?([_a-z0-9-]+)/)?([_a-z0-9-]+)(\.(\/|htm|html))?$ cart.php?url=$5&last=$4&rest

for the / but I am guessing this may get in the way of categories, not sure