How do I pay only for people that play?

On the right side you can find a Settings box.

By clicking on it will show up five options: Archive, Add Scores, Delete Scores, Delete Player and print Handicap Card. In order to choose the Guests for which you don’t want to pay, you will have to select the first one(Archive). 

In the moment when you clicked on it the Status will change from Not Paid in Archived. Thus, you can select from the list for who you want and for who you don't want to pay.

For Unarchiving a guest, you will have to follow the same steps. The only change is, that you will have to select Unarchive instead of Archive.

How do I make the payment?

After you Archived and Unarchived your Guests, you will have to click on Payment Due and fill the requirements about your credit card.