You should have at least two more tables in your database. You might get by with one, but if you plan on selling or shipping product to the same customer more than one time you need two.
Invoices and LineItems.
Each time you sell or ship product (I'm not sure which you are doing here), you create a new invoice record and select the client from a drop down that references the client table. A portal to LineItems is used to record the ID and quantity of each item. The LineItems table looks up unit price and description text from the Products table.
You will then need either a script and/or a calculation to compose the text in the body of your email. It would be simpler to have your database save a PDF of the invoice and attach it to your email.
You might want to examine your templates that come with filemaker pro 11. I think there's one that demonstrates invoicing.