Considering Amazon makes up over a $1 billion of UPS's revenue you think they may just stop accepting Amazon's business even if Amazon starts cutting how much they use them? There's no way. They will keep their business until the bitter end because if they don't their competitors will and Amazon can generate a lot of volume for them.
Why would they start operating the Amazon account at a loss? They would likely lower the volume discount if the volume drops but I don't understand your point.