on the confirm page, I had to update the code for the timestamp from:
<input id="x_fp_timestamp" name="x_fp_timestamp" type="hidden" value="<?php echo("<?php $WACalculatedHash = WA_CalculateHash(''.($WA_Authorize_Net_API_Credentials_Login_ID) .'', ''.($WA_Authorize_Net_API_Credentials_Transaction_Key) .'', ''.$eCartSL->GrandTotal() .'', ''.AuthNet_RandomID() .'', 'USD')?>
<?php echo $WACalculatedHash[1]; ?>"); ?>">
to:
<input id="x_fp_timestamp" name="x_fp_timestamp" type="hidden" value="<?php $WACalculatedHash = WA_CalculateHash(''.($WA_Authorize_Net_API_Credentials_Login_ID) .'', ''.($WA_Authorize_Net_API_Credentials_Transaction_Key) .'', ''.$eCartSL->GrandTotal() .'', ''.AuthNet_RandomID() .'', 'USD')?>
<?php echo $WACalculatedHash[1]; ?>">