Speaking of USPS flat rate boxes, I have combined them before and they were accepted by my carrier. I had 2 used flat rate boxes and the item I was shipping was odd shaped so I cut and taped the two boxes into a weird one that covered the item, and then printed the flat rate label at home. I think the trick here is to give it to your carrier and DON'T go to the Post Office. The anal-retentive-non-customer-service-rule-bound desk clerks will turn you away. At least that has been my experience.
I have even printed flat-rate labels at home and taped them to similar sized regular brown boxes and they've been accepted........by giving them to my carrier.