We don't really have a way to change packaging containers based on quantity, but you could simply set up our "USPS (cheapest method)" shipping using weight-based live rate lookups.
In the settings of each product, enable Shipping and on the following screen specify the weight for each unit of the product, pick a packaging type and specify how many units of the product will fit in that package.
In Seller Admin > Cart Shipping Settings, select your Shipping Origin country and zipcode, select the Shipping Destination countries you will ship to (or leave nothing selected to ship worldwide), and for the same packaging type you selected for the product, enter the empty weight of that packing container. Leave all other settings at 0.00 (unless you want to tack on add'l Handling fees and packaging costs) and click Next to save settings and continue to the Shipping Rules screen.
In the Shipping Rules screen, you should already have a default rule for "USPS Cheapest", so you can just click Back to Admin from there. This will query USPS.com for the proper rate based on the total weight ordered (we lookup First Class vs. Priority and apply whichever is cheaper). This may slightly overestimate shipping if you choose to use a smaller/lighter container than the one you configured, but we don't have a way to change containers based on quantity anyway.