Amazon’s Inverse Shipping Costs

Okay, here’s one that bog­gles my mind. I ordered sev­er­al unre­lat­ed items from Ama­zon on Sun­day evening, part of which con­sist­ed of the following:

• 2- Delonghi Oil Radi­a­tor Heaters weigh­ing a total of 58 lbs. Free ship­ping (cost would have been $44.50) and deliv­ered on Wednes­day afternoon.
• 2- Sil­ver Cuf­flinks weigh­ing a total of 4 oz. $4 for expe­dit­ed ship­ping and deliv­ered on Wednes­day afternoon.

What strange laws gov­ern the econ­o­my of the par­al­lel uni­verse that is Amazon.com?