Shipping Cost Filter

Hi – The documentation for this plugin says you can adjust the shipping cost with the following code:

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add_filter(‘hitstacks_fedex_shipping_cost_conversion’,’fedex_shipping_cost_conversion’,10,1);
function fedex_shipping_cost_conversion($ship_cost){
return $ship_cost+1000;
}
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This is not working for me when I add it into the function.php file. Shipping price remains the same – no error or anything. Just no change. I would like to add a couple dollars to all shipping rates, as the quoted ones are consistently low compared to when we create the actual shipment through our account at FedEx.

Am I missing something? Thanks in advance for your help.