If this happens often, it could be worth taking a look at your client processes.

Here are 4 processes you may want to review:

Detailed quotation

Do you provide detailed quotations that specify the scope of the work and do you talk it through with your client to ensure that they know exactly what has been allowed for?
Do you confirm that they are happy it allows for everything they require?
Do you explain the processes if they require additional work or changes to the scope of the project, as well as what happens when problems occur that are out of your control (e.g. poor weather)?

Contract

Do you have a formal, written contract so that both parties have a clear understanding of the cost and scope of the work?
Do you customise the contract for each project to include clauses that apply to the work and what has been agreed verbally? For example, if the client wishes to provide the materials, do you ensure the contract states what happens if materials are not delivered on time (which can affect your schedule)?

Communication

Do you ensure that you maintain good and regular communication with your client?
Keeping your client informed of any delays or issues arising can make all the difference to the relationship and to you getting paid.
If something important is discussed over the phone do you follow it up in writing with an email to confirm what was agreed?

That little extra

Do you go that extra mile to make your clients feel that you’ve really bent over backwards for them?
Could it be that as part of the service, as well as tidying and cleaning up throughout the job, on completion you hire a cleaner to ensure the client’s property is spotless?
Perhaps you could leave a planter full of flowers.
You could provide some ‘before’, ‘during’ and ‘after’ photos on a USB data stick.