One question a customer recently asked me how to get the corners just perfect on the Box Wallet. It might be impossible to get them perfectly flat but one thing that can help with any corner is to trim notches out of the interfacing.
Just cut a square out of each corner of your interfacing before you iron it to your fabric. You want it to extend beyond where your seam allowance would be to ease the bulk from the corner.
For a standard corner (i.e. NOT boxed) I generally use a 0.50" square since most of my seam allowances are right around 0.25". For the Box Wallet you'll want to cut the interfacing away past where you would box it, so a 0.75"-1" square should help get those corners as flat as possible.