How do I change the currency display from USD to $?
You could try 

hook_commerce_currency_info_alter

For example,

function EXAMPLE_commerce_currency_info_alter(&amp;$currencies, $langcode) {
    $currencies['USD']['symbol'] = '$';
    $currencies['USD']['symbol_placement'] = 'before';
    $currencies['USD']['code_placement'] = '';
}


See commerce_commerce_currency_info for the default currency info implementation.