Freeware TMS can be adapted, for example, to these three business logic options for processing priority delivery orders:
separate route (separate vehicle / courier) for priority orders;
all priority orders are delivered at the beginning (of the route), and the rest in the "general sequence";
priority orders are placed in a general sequence that minimizes / maximizes the criterion, at the execution stage, the possibility of delivering priority orders is continuously monitored, in case of a risk of violation of the planned schedule, non-priority orders are "thrown" from the route.
The logic described above is fairly typical. If necessary, you can implement other - unique, delivery priority processing algorithms.