Poly shipping labels are perfect for odd-shaped mailing because they conform to curved or uneven surfaces. This special material is also tear-resistant and resistant to oils, fats, blood, alcohol and other solvents. Polypropylene labels are heavily resistant to stress. Weather-resistant, moisture-resistant design allows water immersion for 24 hours and endures 40 degrees for 24 hours. Heat-resistance allows the shipping labels to endure 140 degrees Fahrenheit or 60 degrees Celsius for 24 hours.