At present, we have no way to offer buyers a selection among different shipping methods, but we can at least help you configure Shipping settings to meet a specific desired result. We will also be rolling out some enhancements to our Shipping module in the near future, so keep an eye open for that.
You and others reading along here may find it helpful to know the basic formula we follow when calculating shipping:
((cost per oz.) X (total product wt. + container wt.)) + (container cost) + (handling cost)
The (cost per oz.) part would be determined either by a flat per-oz. rate you specify for Domestic or International shipping, or otherwise by standard USPS rates if you have your zipcode entered without any custom rates in Seller Admin > Cart Shipping Settings (we apply whichever USPS rate is cheaper comparing First Class vs. Priority).
Note that you can "game" the formula by applying non-literal figures for various settings, so the formula adds up the way you want. E.g., a common approach would set a fixed Domestic shipping charge as the "weight" of a product or container, then set the Domestic rate as 1.00 and the International rate as 1.50 or 2.00 or suchlike, so the rates actually work as a multiplier for the basic Domestic rate that had been set as a "weight".