Purchases for customers and how to enter this

[bhcs_gnucash at nym.hush.com]

Hi,

just starting to use gnucash for my small business. I am okay at 
the expenses for my business like office supplies out of petty cash 
but I am having problems understanding the correct double-entry way 
of dealing with this scenario:

1) I purchase a part for a customer for $100 and pay the store 
straight away with my credit card. I then wish to charge the 
customer $125 for that part, and also a service charge of $50 for 
work carried out. Later that month my checking account pays off my 
credit card for the $100. I then receive a payment a few days later 
from client for $175.

I'm not sure of the correct way to implement this (how many 
transactions are there). The chargeback to customer is confusing 
me. I have a number of these type of transactions and I really want 
to understand it correctly, before I begin to record them.

2) If we take the first part of the scenario "I purchase a part for 
a customer for $100 and pay the store straight away with my credit 
card." - do I create a new Vendor Bill (creating a new Vendor if 
necessary), then Process Payment? What do I put for the Expense 
Account field in the New Bill step?

Appreciate it if anybody could help with this. I feel if I can get 
this right, I will really be saving myself a lot of headaches down 
the road.

Ken