What you want is really a classic sales orders database design. I would recommend starting by modifying one of the starter solutions that comes in filemakers business productivity pack but some of the designs concepts they use are not very good. If you are looking for a quick fix that may be your best bet. If you want to expand your filemaker skills, follow along with the rest of my post.
You do need a customers table like you stated in your question. You also need the following tables/fields
-orderTotal calculation sum (LineItems::extendedPrice)
-itemDecription lookup from Inventory Table
-itemCategory lookup from Inventory Table
-itemPrice lookup from Inventory Table
-extendedPrice calculation itemPrice * itemQuantity
-ItemCategory Value List of product cadegories. ie apple juce,
Customers --> Orders
Orders --> Line Items
Inventory --> Line Items
Hook everything up and you are off to the races. On you Orders layout you will need a portal showing line items. Be sure to allow the creation and deleation of records from the portal.
Here's a simpler demo file than the starter solution: http://fmforums.com/forum/showpost.php?post/309136/
It uses the same basic structure and method for printing invoices so you may find that after studying this demo file a while, the starter solution may become easier to understand.
Here's a tutorial on summary reports--a basic methd for producing the reports you've sketched out here: Creating Filemaker Pro summary reports--Tutorial
The tutorial thread is old enough that comments/questions posted to it will not pop the thread back up in Recent Items. If you have questions about it feel free to post those questions here or start a new thread and include a link to the tutorial thread in your first post.
Thank you very much guys! I will get back to you, soon as I check everything out.
I have to prepare so I can get back with fresh questions :)