I've searched all over the web and haven't found the answer to this. I'm desperate!
I just upgraded from 1.5.1 to 1.5.4 because of this error and still have the same problem.
It seems to happen only on discounted items. I don't think it has anything to do with sales tax because customers from all over the country seem to be getting this error.
PayPal seems to calculate things in correctly because I see different totals on the PayPal checkout screen than the actual order.
I looked at the logs and it looks like there is also an error 10413, which may occur before the 10426 error.
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