Then you don't understand the economics. Or maybe it's something else you don't understand, but you are clearly missing something. The orbital mechanics of getting to any target in the solar system are unique to that target. Designing orbital trajectories is a profession unto itself.
Yes, it is theoretically possible to have staggered deliveries so that spacecraft arrive at Mars on a schedule other than every two years. But think about it: how would this help? The schedule on which the supply craft arrive doesn't matter. What matters is the latency between when a need manifests itself and when it can be fulfilled. Fundamental physics lets you get to Mars in a matter of days if you could accelerate and decelerate the entire time at 1-G. But you can't, not because orbital mechanics forbids it, but because technology and economics and the rocket equation forbids it.
The one thing you're going to need to optimize for more than anything else is kilogram delivered per dollar spent. That means being as efficient as you possibly can because launch costs are ridiculously high even with reusable boosters. That means using Hohmann orbits, and that means eight month transfer times with launch windows every two years. Yes, those windows are fairly broad, but that doesn't matter. You're still going to have to wait for periods of time measured in months and years for your Amazon order to be fulfilled on Mars.