The handover
Walking the finished work with the customer takes minutes and resolves questions while you are still on site. It also gives them a chance to raise something small before it becomes a call a week later.
Where relevant, explain maintenance or care — customers rarely know what will shorten the life of what you installed.
Invoicing is part of the experience
An invoice that arrives three weeks late, with line items that do not match what was agreed, undermines an otherwise good job. It also delays payment.
Same-day or next-day invoicing that mirrors the estimate structure is read as organized.
The follow-up nobody does
A short message a few weeks later asking whether everything is still performing well costs almost nothing and is remembered for years.
It is also the most natural moment to ask for a review or a referral, because the question has already been answered honestly.