If you have heavy stuff that fits the USPS flat rate boxes they are a good value, but some lighter stuff is cheaper to bag or box, weigh and pay regular 1st class.
Only you can decide.
BTW, dont re-use a flat rate box for anything but flat rate; if its in a flat rate box that is what you pay for. Since they are free, I keep a few at home to 'play my options' and actually know what fits.