We run an automated billing system, and we currently are not able to provide custom debit order dates right now. We can only run debit orders and monthly credit card payments automatically on the 1st of every month (or the first Business day of every month).
You could submit a future dated payment in ClientZone in advance to create a credit on your account, which will offset billing on the billing date. So, let's say you have a product for R99pm. You would need to make the regular payment on the first of the new month, but then on the 15th you could make a future dated payment (essentially a payment to put your account in credit) of R99, which will be offset when billing runs on the 1st of March and you could then repeat on the 15th of March. However, you will need to ensure that if you run any topups or upgrades that you have sufficent credit in your account to allow for any pro-rata or immediate payments debited.
You could also look at signing up prepaid data (if you're just looking at DSL data) on the 15th of the month (you will be billed within 24 hours) and this data will be valid for 12 months, and you can add topups around the 15th to ensure that you have suffcient data (you could also score if 2 for 1 topups are running at the time or if you have Afrihost Plus+).
Hope that helps :)