Unless you're using
a flat-rate plan, carriers charge you based on either the actual weight or
dimensional weight, which is defined by the dimensions of the product. You pay for whichever is greater. To avoid paying extra for unnecessary bulk, consider using
custom-designed poly mailing bags or padded envelopes when shipping soft or non-fragile items.
Thus, if the box is substantially larger than necessary, you're simply transporting air at your own expense.