lightsout714, a TKL keyboard with retail box fits perfectly into a medium flat rate box (the flatter, wider one, not the square one). For keyboards of this weight, it's almost always cheaper to ship by the $10 medium flat rate than to package it yourself and ship it priority. The only exception is if you're shipping within California (or same state), then it'll be a few bucks cheaper to package it in your own box if you're not lazy. This is from person experience. And I'm shipping out 5 TKL keyboards internationally this week, but they're custom packaged because priority flat rate international costs too much. If you need exact weights on a Filco/CM quickfire TKL, here are the exact weights, weighed with a micro scale:
TKL keyboard only: 34.88 oz
retail box + accessories: 11.88 oz
packaging box and padding: ~10 oz (this will vary, depending how box size and density, and what kind of padding is used)
TOTAL weight is 3.55 pounds (56.76 oz). I will post pics later of how nicely TKL retail box fits inside a USPS medium flat rate box.