Many accountancy practices will offer you a fixed fee basis for the services that they provide
The idea is to agree in advance the price for each service that they provide based on certain conditions being met, for example the number of employees on the payroll.
Usually once agreed the whole amount will be divided into monthly invoices, often over 10 or 12 months, and a monthly direct debit paid. This is good for the cashflow of the client as they avoid paying out a large amount once everything has been done and they get to claim the vat as they go along. Many of the utility bills that a client receives are done on this basis and a lot of the software subscriptions are also now being done this way.
At the end of the period, often a year, the fees for the following year will be discussed and agreed and the invoicing and direct debits will be set up for the new amount.
If the services changes during the initial period the invoice and direct debits will be adjusted over the remaining term of the agreement.