How much do you think you could fit in a Medium or Large flat rate USPS Priority Mail box?
I agree with this guy. Have you checked the "If it fits if ships" boxes?
https://www.prioritymail.com/flatrates.asp$15.45 for the largest box. Also, the boxes are free at the USPS (you can even have them dropped off at your house for free). It ships priority, which at it's slowest is 5 business days domestic.