Can I Use My Own Box to Ship USPS Flat Rate?

No, if you could use your own box as a Flat Rate Box it would defeat the purpose of having Flat Rate Boxes as you could just ship any package for $10.95 and call it Flat Rate.  Likewise, you can’t use a Flat Rate Box and ship it as variable weight.  There are free USPS boxes with similar dimensions to the Flat Rate Boxes available, so there is no reason to use a Flat Rate to ship variable weight.  Flat Rate Boxes are available in a variety of sizes and costs.  For an in-depth look at Flat Rate Shipping and when using it is cost efficient see my guide, Utilizing USPS Priority Mail Flat Rate Boxes For Your eBay Business .