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.