If a customer wants 3D modeling work done in addition to 3D printing, the order system should require the customer to pay for the 3D modeling work and then the 3D printing work separately.
I’ve had several instances where a customer wanted something modeled, but then did not go through with the order because it was too expensive to print the object (even though they had been warned this would be the case). So even thought the modeling work is finished and completed, and I have a 3D model for them, I don’t get paid for it. I also can’t just not make the model until they pay for the order, because then I have no way of quoting how much it will cost to print the model.
I think it makes much more sense to allow the customer to submit one order, but if they want 3D modeling done, they have a two step payment system. The first step would be paying for the 3D model based on the time I quote that it will take, and then I complete and upload the model. Once the model is done, the customer can choose to proceed with 3D printing (and the order would complete as they do now) or can opt to print somewhere else/not print at all - but the model is still paid for. Just my two cents, it just gets frustrating to spend x amount of time on a model, then not get paid as I was expecting to.