MarketPress passing incorrect cart value to eWAY

Hello WPMU dev gurus,

I have installed MarketPress and configured it to connect to eWAY in SandBox mode. So far so good, and I can get dummy/test transactions approved.

The strange problem I have is that each transaction is consistently 68 cents short: i.e. if the MarketPress store total is $17.68, when control is passed to eWAY, the Purchase Amount on the eWAY confirmation screen is $17.00.

Once the transaction is approved, the details which are passed back to MarketPress are the new values, i.e. order total is now $17.00.

I've asked eWAY support - they were very helpful - but said they only used values that were passed by Marketpress. They couldn't account for the discrepancy.

Any clues?

Thanks in advance, Ron.