Con,
I'm sorry to report that the Multiple Price upload has a big flaw. It can only add a new price, not update the price. So, unless you can modify the code in function MultiplePricesUpload in the ps_csv class, price upload must be done in 2 steps:
1. delete the price first
2. upload the new price.
By the way, I noted your xls has a column product_delete which has a value 'Y'. This column is not needed although it does not generate any error. If you want to delete a price, the column header should be price_delete.