The box on the left is called the shoe box and as its name implies is perfect for shoes. I use it all the time for that but for many other items as well. The box next to it on the right measures 7 x 7 x 6 and is the perfect size for most mugs.
Next is the Tyvek mailer. They are pretty indestructible. I like them for added strength when sending something in a polymailer especially overseas. If it's going priority I'll throw it in the Tyvek mailer. I could use a box to provide the extra strength but that adds weight.
I use the next two a lot. They are similar in size to a medium flat rate box but cheaper if your item is light than paying the medium flat rate price.
Lastly I use this large box quite a bit. As it's name implies it is a nice large box. It is a little bigger than the Large Flat Rate and almost always cheaper depending again on the weight of your item. You can always compare when you do your shipping.
I never realized how many boxes were available free and how much I would use them once I figured out when they worked best. I usually offer my customer Parcel Select, Priority or Priority Express. On their end Parcel Select is usually the best deal and what they usually choose. On my end with my shipping discount Priority is usually a little cheaper or just a little more expensive. So the customer gets a free upgrade and I get to use free boxes!
Do you use the free shipping boxes USPS offers? Let me know if you have a favorite that I've left out. And Happy Easter. I hope the Easter Bunny left you something good.