To change the wording on the Donations customization

custom_price_string.php

function wtd_subscriptions_custom_price_string( $pricestring ) {
    $newprice = str_replace( 'for 1 day', 'once', $pricestring );
    return $newprice;
}
add_filter( 'woocommerce_subscriptions_product_price_string', 'wtd_subscriptions_custom_price_string' );

function lr_subscriptions_custom_price_string( $pricestring ) {
    $newprice = str_replace( 'every day once', 'once', $pricestring );
    return $newprice;
}
add_filter( 'woocommerce_subscriptions_product_price_string', 'lr_subscriptions_custom_price_string' );