Solved! I had the statements in the wrong order, the first if statement was satisfied so none of the others were checked. Reversed them and voila! I also could have added a <= to each statement
//General Fall back for shipping and cart totals <= 50
$insurance_fee = 0;
$chosen_methods = WC()->session->get( 'USPS_Simple' );
$chosen_shipping = $chosen_methods[0]; {
if ( $woocommerce->cart->cart_contents_total > 500 ) {
$insurance_fee = 7.30;
} else if ( $woocommerce->cart->cart_contents_total > 400 ) {
$insurance_fee = 6.40;
} else if ( $woocommerce->cart->cart_contents_total > 300 ) {
$insurance_fee = 5.50;
} else if ( $woocommerce->cart->cart_contents_total > 200 ) {
$insurance_fee = 4.60;
} else if ( $woocommerce->cart->cart_contents_total > 100 ) {
$insurance_fee = 2.45;
} else if ( $woocommerce->cart->cart_contents_total >= 50 ) {
$insurance_fee = 2.05;
}
}
//The fallback $insurance_fee value of 0 will be used if none of the conditions are met
$woocommerce->cart->add_fee( 'Shipping Insurance', $insurance_fee, true, '' );
}