You can plug in an amount. You plug the percentage amount in decimal format so for 10% you would enter .10
You could change the discount field to a drop down list with a set number of percentages. Just change the field to a drop down list from the inspector, then create a list with your discount amounts.
It still doesn't work. The thing I am learning is that the "discount rate" is in another table from the invoice table. So, I don't really know how they are linked. I put in a drop down menu for discount rates in the invoice date table. But it doesn't give any options on the invoice table. I 'm not sure how they link, how they work together. It seems like a simple issue, there must be something easy I'm missing.
Let me backup and explain the invoice starter solution. The discount is for each line item, so you have to enter an amount in the disc field for each item you want to give a discount. You can give different discount amount for different items. The discount field at the bottom of the invoice is a total of all the discounts and is not editable.
You would have to change the InvoiceData::Discount Rate field to a drop down list from the inspector, then you would create a new list, and enter your discount rates.