From this post I'll tell you how to get a payment receipt in Metro Campus. Since I am a Metro student I don't know how it is done at Malabe or at the other Campuses.
You'll need a receipt when getting printouts, photocopies, depositing money for library or when paying library fine for late returning of books.
You need to go to the 16th Floor of the Metro Campus (Boc Merchant Tower). There is a receptionist and you can ask from where to make payments. At the moment it is the first room to your left hand side, when you enter into the 16th Floor and turn to your right hand side.
You need to provide your SLIIT Identity Card to the officer and tell the reason for your payment (photocopy and printouts, library deposit, library fine.......).and the amount. Then make the payment and get the receipt.
Keep this receipt with you until the balance gets zero ( for photocopy and printout receipts).
P.S - To get photocopies or printouts, you only need to get one receipt (I think the minimum amount is Rs. 250/=).