Flat Rate is convenient, since you don't need to worry about the size or weight of the box... but that convenience comes at a price. If your items weigh less than 20 lbs, Medium and Large Flat Rate Boxes can be much more expensive than using your own packaging and shipping with Priority Mail Cubic instead.
The Priority Mail Medium Flat Rate Box comes in two sizes
If you used your own box that was the same size or even a little bigger, like 14 x 12 x 4 inches, it would be in the 0.4 cubic feet pricing tier of Priority Mail Cubic and can save you up to $4.99, depending on how far you're shipping it. Note that for the furthest distances, and especially if you're shipping to territories like Palau or Guam, the Medium Flat Rate Box can still be a good deal:
Use your own box for a custom size that fits your products, or USPS offers two free "mailing boxes" that are the cubic 0.4 size:
The Priority Mail Large Flat Rate Box comes in one size
Large Flat Rate Box: 12 x 12 x 5.75 inches
If you used your own box that was the same size or even slightly larger, like 12 x 12 x 6 inches, it would be in the 0.5 cubic feet pricing tier of Priority Mail Cubic and can save you up to $9.96, depending on how far you're shipping it. Note that for the very furthest distances the Large Flat Rate Box does offer some savings: