In the Category Type drop down window choose SNOWPLOW
In the Category Description type in : Snow Plow Billing Customer Group
Next bring customers in this group by highlighting them in the right window and dragging and dropping them to the left window or use the arrow buttons in the center. When you have brought all of your snowplow customers into this group click the ok button.
Then when you are ready to generate your invoices for these customers, go to the invoice page, click on generate and choose Other Category. In the drop down box choose your Snow Plow Billing Customer Group. This will onlye generate invoices for them.