ASo up until now all of my cakes have been for my own events or I have delivered them. Cakes have always gone on a cake board that doesn't show and then on a cake stand or this crystal platter I have. I have a pickup on Saturday for a graduation cake. I had planned to make it on a board and put it in a cake box like usual. The customer is coming from over an hour away to pick it up. Since I won't be there to set it out for display should I put it on a larger display type board? If I did that I might have a hard time finding the right size box. The bottom tier will be 12"

You should consider a cake drum. You can pick them up at Michaels or most cake supply stores, and online. 

I buy boxes from my local UPS store and a box that is 14"x14"x14" might work out for you. When the cake board is 12" then I place styrofoam wedges glued to the bottom of the box that will secure the cake board and it won't slip around when transporting. Hope this helps. Oh and I watched a video on youtube about making a cake box incase you might like to watch that too. And last but not least....the person driving MUST drive like a grandma and not a Nascar driver!!!! Good luck 

Thank you so much for the tips! On a cake drum, does it sit flush with the cake or have overhang?

You would usually have an overhang so you, and your customer, can pick the cake up and get it in and out the box without sticking your or their fingers in the cake sides. For a 12" cake, I would usually use a 14" drum (and a 14" box so the cake sits snug in the box).