AndreyPopov
Billing software should allow both onetime billing and recurring billing. Since you cannot predict your customers' financial situations, and they are willing to buy your offline billing software, it would be easier to work with them instead of against. Basically, if a customer is unable to pay everything at once, (one time billing) allows an option for this customer to pay over a certain period of time (recurring billing).
This option will build trust between you and your customer.