Design a UML diagram for an online shopping database system. Keep it simple. In this database, there should be transaction info and item (product) info. Transaction should be the MAIN entity. Transactions can be made by members or guest buyers.

Payment and shipping info should be recorded, if a purchase is made. Product info needs to be sufficient for user to browse and search, Transaction info covers both purchase and return transactions. Show the quantity and unit price of each item ordered. It is also very important to link a return transaction to its original purchase transaction.

Think about a possible case: One bought 1 desk ($100) and 3 chairs ($30 each), charged to one credit card. Later the buyer returned 2 chairs (refund $30x2 in total back to the credit card used).