Merchant Email Receipt detail (MarketPress eComm)

When a user makes a purchase on our site, we receive a Merchant Email Receipt. Part of it looks like this:

========= ORDER INFORMATION =========
Invoice : e0eff2900dc9
Description : Order ID: e0eff2900dc9
Amount : 95.00 (USD)

So the problem is, the "Description" line doesn't really describe anything. It just repeats the Order ID number that's shown in the Invoice line.

This is not helpful in determining what it is that the customer bought. It would be much more useful to have it return the SKU, or the title from the Product page.

The relevant code appears to be here (from authorizenet-aim.php):

// Order Info
    $payment->setParameter("x_description", "Order ID: ".$_SESSION['mp_order']);
    $payment->setParameter("x_invoice_num",  $_SESSION['mp_order']);

Can you please correct this code? Or, failing that, tell me how I can hack it such that my changes won't be lost the next time the plug-in updates itself?