contribution page PayPal error "The link you have used to enter the PayPal system contains an incorrectly formatted item amount"
Well, asking the question seems to have helped me find the answer on my own :-)

Turns out that a custom extension for adding a percentage surcharge was not rounding the result of its calculations to two decimal places. Therefore, the total value calculated and sent to PayPal had more than two decimal places in the cents -- which PayPal doesn't accept. However, the DISPLAYED total amount value on the confirmation page DOES round to two decimal places, so it's hard without looking at the actual PayPal data to see the problem.

The form worked fine as long as all the prices were in whole dollars, but the new price item included cents.  Pure percentage calculations with cents were almost always producing surcharge values with hundredths and thousandths of cents.

Putting in a round() function on the offending calculation solved the problem.

Hope this helps someone else!