Instalment and Usage-Based Billing Explained

Not all recurring billing is a fixed amount every month. Instalment billing splits a known total into scheduled payments, while usage-based billing charges for what the customer actually consumed in a period. Both are common in South Africa — instalments for lay-by, school fees, insurance and financed purchases; usage billing for utilities, airtime top-ups, metered services and per-transaction pricing.

The key difference from flat subscriptions is that the amount can vary, and on debit order rails the amount you collect must always match what the mandate allows.

How does instalment billing work?

An instalment plan takes a known total and collects it over an agreed schedule:

1. The total, number of instalments, amount per instalment and collection dates are agreed upfront.
2. A mandate or card agreement is captured covering the full schedule.
3. Each instalment is collected on its date.
4. The plan ends when the final instalment is paid — billing must stop at that point.

Instalment plans have a defined end, so the billing system needs to track how many instalments remain and never collect past the last one.

How does usage-based billing work?

Usage billing charges in arrears for consumption:

1. Usage is metered during the billing period.
2. At period end, usage is rated into an amount, often with a fixed base fee added.
3. The customer is notified of the amount before or when it is collected.
4. The amount is collected by debit order or card MIT.

Because the amount differs every period, customer notification matters more than with fixed billing: an unexpected debit is far more likely to be disputed. Tell the customer the amount before the collection date wherever possible.

How do variable amounts work with debit order mandates?

A debit order mandate must either state the exact amount or clearly explain when and how the amount may vary. For variable billing this means the mandate should record:

• Whether the amount is fixed, variable or usage-based
• The instalment amount, where applicable
• The maximum collection amount, where applicable
• Any authorised adjustment category, amount or rate for amounts that change over time

DebiCheck makes this explicit: the bank-confirmed mandate carries an instalment amount and a maximum collection amount, and the customer authorises these at their bank. You cannot collect more than the authorised maximum, and changes beyond the authorised adjustment rates require a mandate amendment confirmed by the customer. When designing a usage-billed product on DebiCheck, set the maximum high enough to cover realistic peak usage, but not so high that customers refuse to authorise it. See DebiCheck amounts, frequencies and adjustments and minimum mandate requirements.

Every payment instruction must match the mandate terms. Collecting an amount the mandate does not authorise is a common cause of disputes and reversals.

How do variable amounts work with cards?

Card billing is more flexible on amounts — there is no bank-registered maximum — but the same principle applies: charge only what the customer agreed to. The recurring agreement should describe how variable amounts are calculated, and each charge is submitted as a merchant-initiated transaction referencing that agreement. See recurring card payments.

Instalments vs usage billing at a glance

Kwik supports fixed, variable and usage-based collection amounts across EFT and DebiCheck debit orders and card billing, with mandate terms captured at sign-up.

Related topics

• Billing and recurring payments hub
• DebiCheck amounts, frequencies and adjustments
• Minimum mandate requirements
• How do subscription plan changes and proration work?