All enterprises that are obliged to submit audited annual accounts must have a public auditor. Whether or not you are subject to the audit obligation will partly depend on your organisational form and the size of the enterprise.
There is no law stating that you must use an accountant to keep your accounts. You are responsible for ensuring that your enterprise's account are kept correctly. If you have the necessary knowledge, time and interest, you are free to prepare your own accounts.
If your customers do not pay up, you will soon start losing money. The longer it is since the due date, the more difficult it can be to collect the money. Customers who have difficulty paying their bills will often wait before paying. It is therefore important to follow-up trade debtors continually.