Deciding which box to use for your shipments can save you a lot of money.  There are a lot of different aspects that should be considered based on your product, user experience, budget, and fulfillment.

Durability:  The most important aspect in selecting a box should be whether it will protect your product.  Saving money on shipping and boxes is fruitless if your product arrives broken and needs to be replaced and reshipped.  Corrugated boxes are rated on the bursting test (lbs/sq inch required to burst the box) as well as the Edge Crush Test ((ECT) lbs/sq inch required to crush the edge of a box).  Typical standards are 200# for the bursting test and 32# for the ECT.  If you have a heavy weight or delicate product it is better to go with double walled box or a mailer box which is reinforced by multiple folds.  Indestructo boxes are mailers are essentially uncrushable.  Boxes should have a stamp from the manufacturer indicating their rating, which is helpful to determine which box is the best for your product.  Typically 200# boxes should be fine for most shipments but it is best to evaluate boxes for your specific products.

Size:  Selecting the correct size can save you lots of money in shipping cost.  The aim should be to get you products to fit into boxes with as little open space as possible (be sure to allow enough space for padding and protective materials).  Doing so will decrease the cost of shipping as your product will weigh less, and every ounce counts.  Additionally shipping carriers can charge based on the size of the piece as opposed to the weight.  Dimensional or cubic weight calculations vary by carrier but when designing your box you should evaluate which carrier you will use and what are the most efficient size to use.  When shipping multiple products it is best to identify a few types of standard boxes to use that fit within the dimensional weight classifications that will work best for the carrier you will be using.  Depending on the size of the box some carriers have different requirements for the maximum and minimum dimensions and weights.

Design:  There are a few different box designs you can select from which depends on your product, fulfillment and intended user experience.  Some different types of boxes include traditional corrugated boxes, mailers, tab locked mailers, carry cases, etc.  Traditional boxes are the most efficient to fulfill, but are less secure and are less aesthetically pleasing, whereas mailers, tab locked mailers, and carry cases are more time consuming and difficult to assemble and fulfill but are more secure and look much nicer.

In summary you want to get your packaging to be as light and small as possible while still ensuring that your product is secure and protected.  Aesthetics can be added to the boxes but to save money on shipping and fulfillment you will want to decrease packaging as much as possible.