PayPal EC - which php script decides if transaction total not = order total?

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I am configuring version 2.3.4 and PayPal Express Checkout.  It sends line items to PayPal when the users asks to Checkout with PayPal, but at the final step, it apparently does not send the line items, so PayPal confirms the purchase with the correct total amount, but no line item information.

 

I have read on the Forums that if PayPal Express Checkout discovers the transaction total does not equal the Order total, it sends PayPal the total amount but not the line items.  Apparently this can sometimes happen if a tax or shipping module gets things out of balance.

 

I cannot find where that code to test totals is located.  I would like to test by forcing it to send the line items anyway.

 

Does anyone know where this check is done?  It would be very helpful to me in solving this issue. Thanks.