I have the following formula:

if(date-TODAY()<=7,"Payment due","")

The problem is when today's date falls after the first date the result is a
negative number and it returns "payment due" since it's less than 7 even
though the payment has been made. A solution (since I'm a new user, not
likely to be the best one) might be specifying a range of from 1-7 days but I
don't know how to do that.