"Support and maintenance" is the vaguest line item in software contracts. In a software support retainer, that vagueness is expensive for both sides: the client expects everything, the supplier budgets for nothing, and the relationship sours the first time something breaks at an inconvenient hour.
Four things a useful software support retainer specifies
What counts as an incident, and what does not. The site being down is an incident. A feature request is not. A bug that affects one user in an unusual configuration is somewhere in between, and both sides should know where.
Response time, and what it means. "Four-hour response" almost never means fixed in four hours; it means someone acknowledges and starts. Say so explicitly, in the agreement, or it will be discovered during the first outage.
Hours of cover. Business hours in a named timezone, or genuine round-the-clock. The second requires a rota of several people. A small studio promising it is promising something it cannot staff, and the promise fails on the first bad night rather than gradually.
What routine work is included. Dependency updates, security patches, certificate renewals, platform version bumps. This is the part that actually prevents incidents, and it is the part most likely to be quietly skipped when nothing seems to be wrong.
What it should cost
A useful software support retainer for a small product is a few days of work a month. If it is much cheaper, nothing is being done routinely and you are buying an insurance policy with a large excess. If it is much dearer, you are paying for capacity you may not need — worth checking against simply hiring an hour of help when something happens.
When not to buy one
If your product is stable, rarely changes, and you have a competent developer who can deploy, a retainer may buy nothing. In that case, an arrangement to call us when needed is more honest and cheaper for you.
We would rather say that and keep the relationship. A retainer that quietly does nothing gets cancelled within a year anyway, usually along with the goodwill that came with it. What ours covers, and what it deliberately does not, is written out on the page for our application support and maintenance services rather than left to the invoice.
On the same decision, from another angle: rolling back without making it worse.