Tape + Sharpie becomes easy-pull labels for bottles, jars and cups
I tried a couple of different labeling systems for pumped milk when I went back to work, and none were easy to put on and take off or re-label. I finally figured out a go-to method that works for bottles, jars, and any other plastic or glass-surfaced container that I've tried it on. I take a piece of my clear office tape (though I would think masking tape would work just fine for this too), and turn under one side so that it has a small tab on one end. I stick it on the bottle or jar, label it up with a permanent marker, then easily can pull it back off when I'm washing the bottle later. No fighting yucky stickiness or going crazy with my fingernails to get the label off. It works every time, is easy and quick.
I'm a big fan of the label maker, but for one-time labels? This is perfect!
(A side-hack, if you will, is that I often store my breast milk in half-pint canning jars, which stack well, you can freeze [as long as you leave enough room at the top], they don't leach anything scary or retain odors, and they're totally affordable. We've not had any breakage problems, and I've been using them for at least six months. I transport them to and from my daughter's caregiver in one of those neoprene Built NY bags, so that helps cushion any falls between home and there.)
If you decide to store milk (or anything else) in canning jars, pick up a package of Ball plastic storage caps. They fit the glass jars, are easy to wash and handle, and go in the freezer.