Marketpress Paypal Problem

Ok, have been using the lite version now for a little while and things seemed fine, so upgraded to the full version today and now it won't complete the transaction with paypal citing...

"10413 - Transaction refused because of an invalid argument. See additional error messages for details. - The totals of the cart item amounts do not match order amounts."

I am right in the middle of a voucher site campaign and decided to deactivate the paid for marketpress and re-activate the Lite version.

Went back to authenticating correctly with no problems... Bug?

Am i doing something fundamentally wrong? Installed full version as advised

please advise urgently.. thanks

http://www.bay-city.co.uk

Thanks