[tasn]

I understand it may not have been very clear. Though the point is that no one cares about delivery order, what they really care about is processing order. So it doesn't matter if you ensure delivery order if they process it out of order.

As for relying on the customers to get ordering correctly: it's actually more involved and easier to get wrong than people realize, and it's better to avoid it altogether in how you design your API if possible.