Ubercart has problem working with Pay Pal WPS module.
INTRO from doc: 'submit': When a sale is being completed and the customer has clicked the Submit order button from the checkout screen, the hook is invoked with this op. This gives modules a chance to determine whether or not the order should be allowed. An example use of this is the credit module attempting to process payments when an order is submitted and returning a failure message if the payment failed.
see [url]http://api.ubercart.org/api/function/hook_order/2[/url]