If I understood you correctly, you are just trying to add new fields to the invoice form and then use them in your form. you should first go and add the field to your tables, which you would do by going to Tool --> User Defined Fields --> Manage User Fields. There are different documents or categories given. For ex. for invoices, Sales Orders you would add your field under the Marketing Documents. If you want the field to be just one per invoice, add it to the Title, otherwise if you want a field per invoice or Sales Order line, add it to the Rows section. Once you have done that then you can just create a edit box or drop down to represent the field and set the datasource for that to your field. If you want example code to do that, let me know.
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