You can either configure products for shipping multiple units in a larger box and then just use a smaller box when warranted, or configure for shipping single units in a smaller box and then just consolidate into a larger box when warranted; either way, you'll sometimes wind up a bit ahead, so you can refund or just pocket the difference.
At present, our system is only smart enough to consolidate different products into the larger product's container when the Tube package type is used (as for posters) -- if you order large posters in a large tube along with smaller posters, the system will consolidate the smaller posters into the large tube. Since that configuration is done "behind the curtain" (buyers don't see which package type you are using), I don't think there's any reason you couldn't configure your boxed-shipment products with the "tube" container type, so ordering a larger product would then allow smaller products to be consolidated into the same large package container, even though you're actually shipping in boxes.
