It's current not possible to combine orders (more than changing the 10x multiple of the same board). Adding combined orders would complicate the integration quite a bit with the manufacturers. Nothing I have in the todo-backlog at the moment.
But I agree with you that the shipping cost becomes a bit high when doing two separate orders (that later manually gets combined by the manufacturer). They all got their own way of calculating shipping cost. When testing, they seems to add $2-5 when increasing quantity by 10 on a single PCB order.