For future reference, two tips I've used in the past:
- bike shops have free boxes, and often will pack/hold for a pretty reasonable fee
- Amtrak is a great deal for domestic bike shipping, provided you can find a checked-baggage station near you. For bikes in particular, their boxes are very roomy and require minimal disassembly (pedals and handlebars, which on recent bikes can typically be done with just an Allen tool). They even hold packages at the receiving station for >30 days for free.