;) I paid shipping cost for my customer, but I will get it back from my customer. How do record the transaction.

Make the customer sign some sort of receipt book.

XenoSapien

Reverse the original entry, so if you record the entry as follows:

D/ Shipping/ mailing (expense account)

C/ Bank /Petty Cash


Reverse it as follows:

D/ Bank / Cash


C/ Shipping/ Mailing