When going to the start of my tour I have had good luck with packing my panniers in a cheap used suitcase from a thrift store ($6-8). I then discard the suitcase at my destination. If you are flying home from a large city you can find a thrift store and buy another.

I have also used a duffel bag that was within the allowable size for checked baggage. I can typically fit everything in one bag, but if it doesn't fit some can go in a carry on sized duffel. I founds some inexpensive ones that were the correct sizes and fold up very small at walmart. http://www.walmart.com/ip/Expandable...i_sku=11069689
http://www.walmart.com/ip/20-Expanda...i_sku=11069683

The expandable duffel bags could be carried along, but I typically would mail them to or from my tour so as not to have to carry them the whole way.

Not sure how much you carry, but I find that with 4 smallish panniers I have more space than I could use. Maybe it would help is you picked gear that nested better or packed more stuff inside other items.