for this specific task, i sell batched hours, usually in 8 hour blocks (being a full working day). i always say upfront how long i think tasks will take, and re-invoice when there is 2 hours or so left, depending how quickly the client burns through the time.

this is the only one that has worked for me and kept my clients happy too. i used to do monthly billing, but clients often felt like they were paying more than they should, and the reality of it is that with the batch of hours, i end up invoicing more frequently and getting paid more, for the same workload.

I'd recommend and use a combination of both. Since you want to

A) deliver great service. but B) don't do a ton of free 'support' work that eats up your time (or your teams' time). (or spend days negotiating rates and such)

We bundle a bunch of services (hosting/backup/maintenance) into one contract. Including helpdesk support up to say 15 or at most 20 minutes free (the contract says 15 and fair use). This way we can add value to the contract that in reality won't cost us much (shorter calls 'halp I forgot my password, my 50MB image won't upload' usually weren't billed anyway). Some folks call a lot, some don't call at all, some only during certain seasons when their business is booming or they update their content. So in our case it evenly splits the costs of the 'free' helpdesk among all our clients and we can also cover the costs of the software (Freshdesk etc.).

Anything that takes more than 15 minutes (actual development or design) is charged hourly at a slight discount from not having the contract. Normally we will respond within x-hours, and fix/do things within x-days. If a client has an urgent request we can start within 24 hours but the hourly rate goes up by say half.

Batched hours I would recommend only for 'larger' clients (when you're sure they'll at least have 2000 euro/dollars worth of additional work a month). Otherwise when their own budget is tight or they don't use the budget they'll request it back or they feel 'overcharged' since you didn't do anything for them.

With larger clients this is usually not much of a problem since money isn't their primary concern but getting budgets approved and happy management is, so if the 2000 a month is approved by management the lower-level employees don't care if its used or not, since the money has already been 'spend' in the books each month and management is happy when things run smoothly (which is often the case when the money is there). If the budget hasn't been spend you can offer them some extra service to sweeten the deal. Like, 'hey there's still a quarter of the budget left, how about I use that to neatly present your site statistics in a presentation and research some improvements for you to discuss with management next week?'. Sometimes they say 'cool!" at other times they say: 'you guys already did such an awesome job, why don't you take that as a bonus'.

In closing, as designers/developers we already do a ton of work for 'free' (improve our skills, keep up with the trends, fix our own dumb coding errors and PC crashes) that we can't afford not to charge for the time we spend doing work for clients.

We have monthly retainers with clients for X number of hours.

Agency I worked at had a simple structure:

Support contract, x hours per month (on a lose it or use it basis).

Hourly rate for clients not wanting to commit to a support contract

If it was deemed too big a task, based on advice from the agency, it would be billed as a new job.

I like to think of it more as a reservation of work. The client pays X to reserve X number of hours from me each month. If they do not use all the hours, that's simply the cost for reserving my time & making my self available to them. In the event that I go over the estimated hours, I simply bill them for the extra.

I use an exhaustive Service Level Agreement contract, which details exactly what the client receives for a monthly fee. In a nutshell they generally cover uptime support, backups, general maintenance/bug-fixing, and user/admin support.

As example, one client gets 4 hours of one-to-one support included in their SLA. Basically to do standard CMS activities which they could actually do themselves.

If anything is more than a simple change, we draw up a new project for the work.

This method ensures I have guaranteed income from several sources per month at a fixed rate. These retainers enable me to take holidays and forecast easier.

We work in two ways, and leave the client to choose which billing method we use:

• The first option is hourly, where we collect all the hours spent and bill the client at then of each calendar month. Client's who take this option are usually more tech-savvy and capable of managing their site on a day-to-day (some with an in-house web or IT team, others that just know their shit).
• The second is by way of a maintenance contract, where we agree a singular monthly cost based on the level of service they think they need. That runs for 6 months, before we review the cost and (usually) the client signs up again. This is usually for clients that are less comfortable with running a website/CMS.

PS. It's worth mentioning that we're a smallish design agency, and our clients range from local businesses with just a few employees, to multi-nationals with large numbers of employees and technical resources.

PPS. When I worked freelance, I billed everything that wasn't fixed cost in batches of 10 hours. That worked great too.