You need a summary field. Check Running total & Restart summary for each sorted group and then sort on category. You will have to perform a find on your client or a portal to limit summary to your current client not all clients.
It's not that simple if you want one row for each product with a subtotal displayed, but it can be done if you work at it. On the other hand, if you have FileMaker 12, there's a simpler approach that uses the Execute SQL function returning values in a multi-line field and no portal used.
For using a portal: Sum_Calculation based on condition
Using Execute SQL: FMP 12 Tip: Summary Recaps (Portal Subtotals)
Feel free to post follow up questions here if you have questions on how to implement either approach.
Option 2 is really amazing.
Thanks for the help.
Now I have one last question. After taking Option 2, is it possible to use the filtered value for a new calculation?
If I try to take the value, it always takes the unfilted value. What solution is there to take as the calculation base, the value after having applied the filter?
Option 1, a filtered portal with a summayr field, is a "display only" method. You'll need to use one of the other options if you want to use the sub total in a calculation.