There's a bug in 4.3 where prorata services ordered on the prorata day can come up with a price of 0. The service is scheduled to renew the same day, so the customer would be invoiced shortly for the second  month. If you use prorata settings in your packages, this issue is resolved in 4.4, however here's some steps to resolve now if you need to. Be sure to back up the file first, and be sure to be running 4.3.2. You should be able to patch this issue yourself by making a simple file change