Pasty butt is common in some shipped chicks. It is caused by dehydration, getting too cold or too hot, and is serious if not discovered and fixed. I usually see it in a few with chicks in the mail.
Wash the vent area with warm water and soap rubbing the poo off. Offer electrolytes or a lot of water, dip the beaks for a second several times, and make sure they continue to drink. Use a thermometer on the floor of your brooder and keep it 90 F the first week, and alway have a cooler area to get to. Decrease temp by 5 degrees each week.
Look at vents every day since pasy butt may keep happening for up to a week or 10 days. If chicks appear weak or lethargic, offer more fluids. Good luck.
