The field you select while creating a sub-summary part is the field on which you will sort your records so that records of the same value in this field will be grouped together under/above the same sub-summary part. You can put any combination of fields that work for you into the sub-summary and none of them need to be the field you selected as the sort field.
The obvious answer is to make your payment coupon part of the sub-summary part that you've already created if both are to be based on the same sort field. I suspect that some aspect of your layout design is preventing that. If so, then describe your layout in more detail and maybe someone can suggest a fix.
Hey, that was a very quick response. I've discovered that there is already a Trailing Subsummary part in which I can put the fields and it is working now. Thank you very much for the assistance.