Sounds like a chicken and egg scenario...
First question: Why select a different itinerary based on the total bulk plan volume? It could very well be that your business scenario can be dealt with in a different (easier) way in OTM.
One way of solving your scenario is to have the orders available for bulk plan analysed (i.e. check if sum of volumes > 10 CBM) and assign an itinerary based on the result. So, assuming you run your bulk plan automatically in the morning using an order release saved query, you can set up another process 10 minutes earlier that starts an agent for a single order of that same query, calculates the total volume of all orders, and then updates the buy itinerary for those orders. Note however that you may not always get the expected result, as some orders may not be planned on shipments. Also, if you want to run the bulk plan manually, my solution won't work either.
If you describe the business scenario you want to solve using OTM, we may be able to help you better.
Lourens