addManyIssue

<?php

$feeds = array(
    array('transaction_id' => 123,'customer' => array('...customer data...'),'details' => array('...line items...')),
    array('transaction_id' => 456,'customer' => array('...customer data...'),'details' => array('...line items...'))
);

foreach($feeds as $transaction) {

    $detailTrans = array();
    foreach ($transaction['details'] as $k => $detail) {
        $new_detail = $modx->newObject('FCartTransactionDetails');
        $new_detail->fromArray($detail);
        $detailTrans[] = $new_detail;
    }

    $customerTrans = $modx->newObject('FCartTransactionCustomer');
    $customerTrans->fromArray($transaction['customer']);

    $newTransRecord = $modx->newObject('FCartTransactions');
    $newTransRecord->fromArray($transaction);
    $newTransRecord->addOne($customerTrans);
    $newTransRecord->addMany($detailTrans);

    $recordSaved = $newTransRecord->save();
}

// Results
//FCartTransactions = 2 records
//FCartTransactionCustomer = 2 records
## FCartTransactionDetails = 3 records
#1	82329803	2013-06-12 06:57:49	GB Logo Shirt	16.00	1	0.510
#2	82379757	2013-06-12 06:57:49	GB Logo Shirt	16.00	1	0.510
#3	82379757	2013-06-12 09:25:10	GB Logo Shirt	16.00	1	1.000