Unfortunately this is how Amazon work. Even if you do select group items together there is no guarantee that they will come as one package. What would be nice is if there was an Aramex option that would allow you to batch a weeks worth of deliveries in one shipment. But that is just wishful thinking.

Also get Aramex to check the weight in front of you when you go to collect your items. I have caught them a couple of time slightly increasing the weight to move you into another .5kg