Hi,
When you say you want to enter customer details what kind of details you want to enter, in fact you can maintain all piece of information in customer master record and fetch the same in mail copy which you want to send to your customer via output config with some development. If you fell still you required some additional details anyhow then you can use text config for the same and values can be maintained.
For expense details you can maintain the same at condition level with help of condition type as per your requirement and same can be fetched on your mail copy via output config with some development.
Many Thanks.
Sumeet Sah