Standard Online Sales Sequence

The sequence in brief

1. Customers view your online catalogue
2. Customers add items to a shopping cart
3. Customer proceeds to check out
4. System calculates freight and sales tax, if any.
5. Customer pays through a payment gateway (or one of other payment methods)
6. Merchant receives an order notification and payment reciept by email
7. Merchant ships the goods.

From your customers perspective

It is helpful to view things from your customers perspective, this is how it might proceed from their point of view 

1. The customer may click a product classification link first
2. The customer may search or browse your products
3. Then click on a product title for more information
4. Click on the "buy" button to add this product to their shopping cart
5. Change the quantity and press "click update"
6. Continue shopping perhaps adding additional items to their cart, or...
7. Enter a delivery address
8. Tax and shipping charges are automatically calculated, if any
9. Enter remaining order details (you may collect information about the order such as size or colour) 
10. They the choose their payment method (an order will then be emailed to you the merchant)
11. The customer then completes the payment by their chosen method (If the customer choses "credit card", then a secure form will appear to collect credit card details)

After you have received confirmation of payment you can then ship the goods

Once you have set up your shopping cart system it is a great idea to navigate through the above processes. 
Is it smooth and enjoyable?  Are their any changes you think could improve it? 
Testing and becoming familiar with your own system will enable you to understand and thus effectively communicate with your customers