I have been working all day on some workarounds. The order does go through now, but when examingen the logs things are not yet in order.

I have figured though that to do this correctly you need to calculate all values excluding tax and put all the tax in TAXAMT.
If DISPLAY_PRICE_WITH_TAX=true then you need to remove the tax from the amount of the items, shipping and other order_totals modules.
Next to that you need to handle the credits properly. For instance by substracting it first from the items, if that is gone to 0, substract the rest from shipping, handling, etc. Till all credits are used. The key here is to not have anything go negative. But wat about credits and tax?

I am running out of time to do this correctly so I am going with just putting the total amount into paypal (thats all that shows in the express checkout anyway!) by unsetting all but $options['AMT']

For those who would like to continue the work that I have done, I have attached the paypal express module.

I have put the comments
PHP Code:
//// bof --> 10413 fix 2008-08-27 jabbah.co.cc 
around my changes
HANDLINGAMT should never be negative!

PS rename your old file to paypalwpp.orig.php or something and keep it as a local backup copy. You probably will need to remove it from your server and put this file in its place.

Good luck!