Note: This same process can be followed to modify any field on the A/R invoice form.
When you are in the forms editor, select the form that you would like to edit, and you should end up on a screen something like this one:
Right click the quantity field that you would like to change, and select Settings:
In the settings box for that field, copy and paste the following text into the Format box: #,###.00;; ;
Hit the Accept button and repeat for each of the quantity fields on the form. When you are done, click the Save button to save the changes to the form.