I learned this trick when I worked in warehousing logistics. thought I would share this. this helps keep costs down for you and your buyers.
you can check with carriers, they offer a huge range of box sizes for free along with padded envelopes and labels. that's why you should not modify carrier boxes because they offer a huge amount of different sizes. However, if you need to ship first class (under a 13oz.)you may have to modify a box. it's best to modify plain brown ones or wrap in brown packing wrap *check with carrier prior to shipping to make sure appropriate labels and documents are completed prior to shipping(depends on where & what u ship) FRAGILE, batteries,ECT.
*as another helpf...