Shredded lettuce is already spoiling when you get it, because the structure of it has been damaged by shredding.
If you are going to keep lettuce, you're better off getting baby greens- that way, what you have is essentially a whole plant, just sheared off at the roots. Less damage, keeps longer. or buy a head of lettuce and take off a few leaves to shred at a time.
Store the rest in a ziploc bag with extra air in it, and half a damp but never wet paper towel.
Use up all the shredded lettuce now, or give it to someone else to use up. freezing it will mean it thaws out rotted. Shredded lettuce is good for about 10 minutes, and then it's a waste. Learn how to tear up your own.