a member from my home stae of wisconsin requested a book from me on 8/2/18. i mailed the book out the next day 8/3/18. the USPS tracking has a message that i have never seen before on the day it was delivered 8/4/18. this is what the tracking says instead of delivered. Held at Post Office, At Customer Request. what does this mean.

i waited until Tuesday to send the member a PM about this which included the tracking number info. haven't heard back from the member and the message i sent has not been read.  according to the tracking info page they have attempted delivery for 3 days now with no success. has anyone ever seen this message before or had this happen to them? i would love to know what this means as it is new to me. thanks for any info. 

If it's held at customer request, she put a hold on the mail. She probably went away and will mark it received when she gets back.

Yep, what Bugg said.