Walmart has developed a set of shipping guidelines with the goal of maintaining the integrity of their marketplace, improving their brand image, and cutting down on buyer confusion.
Under their shipping policies, branded boxes and marketing inserts are prohibited. Generic packing slips are allowed, but you can't brand them.
Walmart also prohibits shipping orders through Amazon FBA.
For the full list of shipping guidelines, please see Walmart's partner knowledge base article Walmart Shipping Practices Policies.