To add a "Total weight" line to the shopping cart page, change lines 442-454 of order.php from:
$smarty->assign(array(
'token_cart' => $token,
'productNumber' => $cart->nbProducts(),
'voucherAllowed' => Configuration::get('PS_VOUCHERS'),
'HOOK_SHOPPING_CART' => Module::hookExec('shoppingCart', $summary),
'HOOK_SHOPPING_CART_EXTRA' => Module::hookExec('shoppingCartExtra', $summary),
'shippingCost' => $cart->getOrderTotal(true, 5),
'shippingCostTaxExc' => $cart->getOrderTotal(false, 5),
'customizedDatas' => $customizedDatas,
'CUSTOMIZE_FILE' => _CUSTOMIZE_FILE_,
'CUSTOMIZE_TEXTFIELD' => _CUSTOMIZE_TEXTFIELD_,
'lastProductAdded' => $cart->getLastProduct()
));
to:
$smarty->assign(array(
'token_cart' => $token,
'productNumber' => $cart->nbProducts(),
'voucherAllowed' => Configuration::get('PS_VOUCHERS'),
'HOOK_SHOPPING_CART' => Module::hookExec('shoppingCart', $summary),
'HOOK_SHOPPING_CART_EXTRA' => Module::hookExec('shoppingCartExtra', $summary),
'shippingCost' => $cart->getOrderTotal(true, 5),
'shippingCostTaxExc' => $cart->getOrderTotal(false, 5),
'customizedDatas' => $customizedDatas,
'CUSTOMIZE_FILE' => _CUSTOMIZE_FILE_,
'CUSTOMIZE_TEXTFIELD' => _CUSTOMIZE_TEXTFIELD_,
'lastProductAdded' => $cart->getLastProduct(),
'weightUnit' => Configuration::get('PS_WEIGHT_UNIT')
));
and then add the following at line 100 of shopping-cart.tpl in your theme's directory (after the {if $shippingCost > 0}):
{l s='Total weight:'}
{$cart->getTotalWeight()} {$weightUnit}