Labels on a bar that is still losing water
Updated
A soap bar is an unusual label substrate: slightly greasy, slightly alkaline, and still exchanging moisture.
A surface that moves
Even a cured bar continues to lose and gain water with the humidity, and it carries free oils at the surface from the superfat.
Which means an adhesive label applied directly to soap frequently lifts, and it lifts at the edges first where the customer will pick at it.
And the bar shrinks slightly over time, so a label that fitted at wrapping can wrinkle later.
Adhesive and stock
If you are labeling the bar itself, ask the printer for an adhesive suited to a slightly oily surface, and expect to test several.
Synthetic stock rather than paper, since a bathroom is humid and a paper label on a bar in use disintegrates within days.
Apply to a fully cured, wiped, dry bar. Applying to a bar that is still sweating is applying to a wet surface.
Bands avoid the problem
Which is a large part of why the craft uses them.
A cigar band is a loop of paper held by its own overlap or a small adhesive spot to itself rather than to the soap, so the soap's surface is irrelevant.
The same is true of a box, a sleeve and a tag.
So the label adhesion problem in this property is usually solved by not adhering anything to the product, which is a genuinely different answer from every other property in this estate.
Testing it
Wrap several bars and keep them: one in a warm humid room, one in a cool dry one, one in a bag.
Look at them at two weeks and at two months.
And put one in an actual bathroom, because that is where the label ends its life and where paper fails fastest.