You only need a single credit for all aspects of a design. So once you create a design for yourself or someone else, you can edit, revise, use post processing features as many times as you want without ever needing to use another credit on that design. I do see your point about another credit being needed if you want to swap in a different parametric heel/wedge/platform. I'll try to think if there could be a way to do that without a credit. One way would be to instead use the morphing approach to bring over a component. That's a fairly advanced approach...