With Amazon Prime (required for this, at least for now), you pay one time per year and get free two-day shipping for the year (along with other benefits like Amazon's Netflix-like video service and free delivery of diapers/nappies). With Prime, you generally don't think about combining shipments, since there is no benefit in doing so.
Except for those intangible benefits like reducing carbon footprints. I tend to feel guilty purchasing one-off small items and tend to group those purchases, even though I don't have to.
Amazon spends significant effort to join separate orders into combined shipping units for reduction in cost and impact. It's clearly in their own best interest to do that. I've experienced multiple times where I've placed more than one order on the same day and they show up at my door in the same package.
The cost is at least another box to fold, move along a conveyor belt, fill with padding, attach a label, route for shipping, gas/diesel/electric energy to deliver from end-to-end and waste to recycle.
Those aren't costs I pay. Anything it costs Amazon is absolutely meaningless to me; I've already paid for it in my Prime membership, and I already brought up the point about the intangible costs.
Except for those intangible benefits like reducing carbon footprints. I tend to feel guilty purchasing one-off small items and tend to group those purchases, even though I don't have to.