Make sure you remember the exact date and time you first notice it, also weight of the box and any witnesses..
If the discomfort you feel now becomes worse when you lift a box in the future, report it immediately!
If you do not, Liberty Mutual might try to claim you did it outside of work.
If it was another employee who told you to report it then it is possible if you have a dispute with Liberty Mutual they might get a statement by that employee, so be aware.
It is always best to report an injury immediately. But I believe you have up to 30 days by law to report it.
Insist that an official first report of injury be filled. Request and obtain a copy of the first report of injury. Insist that you see a doctor. They will send you to a UPS doctor. Do not allow the UPS manager to have a "conference" with you and the doctor...Tell him to go away and you will inform him of any restrictions the doctor sets up. Report any work restrictions immediately. You also have the right to see your own doctor in addition to the UPS doctor. I would see my own, the UPS doctor is paid by UPS and can often be biased.
Take copious notes, every conversation, every appointment, miles driven to doctor, missed work etc. etc. Keep copies of everything. Do not sign any agreement with Liberty Mutual before asking an attorney.
Remember this injury could last a life time. If you don't report it as happening at work it may cost you hundreds of thousands in the future.