It's best to be sure, because as Phelix Unger says, you know what happens when you AssUMe.....
If the customer experiences shrinking after the shirts are home, they will not be happy... maybe check with FOTL to find out if that's what the PS stands for, because I've had shirts from Old Navy and Target Shrink and it looks terrible! I bought them b/c my market seems to like Old Navy and Circo for girls decorated tees, but can't do it with the resulting shrinkage...
So - if you cannot verify the PS is pre-shrunk - either let the customer know you are unsure if they are pre-shrunk and explain what will happen if they are not... let them decide whether or not to proceed (and not hold you responsible), or ask them to bring you preshrunk that is labeled as such, or ask them to use the shirts you supply as you know and can verify the quality results... it will be worth them buying the shirts from you for a quality product you can count on...
The last thing you should do is just guess and end up being wrong with an unhappy customer coming back to you, when you saw it coming as a possibility before anything happened...
Good luck to ya!! Hope they're pre-shrunk...
As a side note, I just received a request for a few designs on a shirt and the person supplied the shirts, FOTL I think, but I do know it did not say pre-shrunk. So - being friends - I just asked if I could wash them and dry them on high to shrink them myself if they are not pre-shrunk... I mentioned if they are not pre-shrunk, that might not do the trick entirely, but if they are not pre-shrunk and I don't, the shirts will be ruined after the first wash...
Play the cards straight, the customer will learn to rely on you to take care of them...
Again, best wishes...