Guaranteed 3-5 Items here's the possible items:
 
Collapsible Running Cup
Sticker
Texas Outlaw Merchandise (T-Shirt, Longsleeve, Hat, Beanie)
Shoe Bag
"Break The Limits" Flag
Random Race Tee
Sticker Pack
Bracelet 
 
The Mystery Box is not guaranteed to be a box, nor the box seen in the photo above. It can arrive in a box or a package. No items are guaranteed and are completely randomized. Each box will have a few or several of the items above.