As an e-merchant, you know that shipping costs are a budget to take into account when calculating your margins. And it’s sometimes possible to considerably reduce these costs by offering slower delivery methods.
But still, as an e-merchant concerned with customer satisfaction, you don't consider this option for fear that the shopping experience will suffer.
Studies show that nearly 90% of consumers are willing to wait longer for their order in exchange for reduced shipping costs. So this is a strategy worth considering. And don't forget that you can offer several delivery options for the same area, with longer or shorter delays, and therefore more or less expensive.
Note, however that the rarer your product is, or the bigger it is, the more your clients will be inclined to wait a few more days to receive it.