A direct debit can be used for a variety of payments.  Fixed payment amounts allows your customer a manageable fee that may be the difference between your business making the sale or watching the customer go elsewhere.   It also provide them with some certainty and the ability to budget for a fixed amount at the frequency agreed to between your business and the customer.

Some direct debit providers provider an "on demand" billing service.  This means they can handle variable payment amounts at varying times.  This may be a very useful feature for your business; remember - flexibility is key to any successful business. If you are thinking of giving a direct debit authority for a variable amount you should make sure that you will get a bill first so that you will know how much is to be debited. You may also want to ask the merchant whether your authority can be capped at a maximum specified amount.