What is the actual amount of time that you invoiced for? You have to know some more on the invoices or research some more on it.
I agree Bayliss, it should be on a fixed prize project. If the provider is confident and you both had a clear instructions then a fix prized project would be a good deal for the both of you.
Billing style is up to you. I use fixed-price for most things but some of my clients seem to get along better with hourly billing.
The main issue has been said -- manage your client's expectations. Make it clear how much things cost and how long they take.
Of course, if you tell your client "no problem, that's a two-hour job" and then take seven hours to do it, you may have to suck it up on your end. If you grossly underestimate something and quote too low, you can't really go back and ask for more money. Even if the estimate says that it's just an estimate, asking for much more than you estimate could make for an unhappy client. Sometimes you lose a few bucks to learn the lesson.
If anything, pad your estimate and then come back with, "hey, guess what it took me an hour less than I thought, so you save $XX today...".
If we possibly knew what you were invoicing for (time and total bill), we could maybe see how you'd find compromise.
But, based on the knowledge we have, this is a reminder of why I got out of this business. Arrangements between a designer and the employer is always tough, and communication has to remain HIGH.
If he wants you to do updates, you say that this is the expected time, which will be billed this much. Immediately point out any issues you see with the updates, and how much extra time they could take. Basically, propose a budget for everything you do. And, if you can, get some money up front.
I say point out possible issues first, as people hate hearing of problems later, as it makes you look like a weak designer trying to get more in order to "learn" more about your field.
Point everything out, and explain the costs. Confusion can be disastrous.
*I wouldn't go legal on this one. I'm sure these costs couldn't have been that much based on three "small" updates. You want to waste time and money while also offending a client who seems confused as is? Not a good policy. Blame yourself for not being clear, compromise on the price, and continue on. Referral business is good business.
Not sure if was mentioned, but you should politely explain what went into doing the work he requested, and why it took the length of time that it did.
Also you can apologize for the fact that the bill's size was unexpected and offer to revert the changes if he is unable to pay for the services provided.
It's always good to informally provide a rough estimate as well as let people know what you're hourly rate is to avoid situations like this.
I work with a minimal charge of 1hr no matter what the work is. Before I start anything I make sure I have an email exchange with the client and that he understands the fee.
It is unpractical to draw up a contract for every simple update, specially the easy ones since you will spend more time with the contract than doing the work.
What I may do for long term clients is to do an "easy" update and simply wait to tack the time to the next larger update, but this is not advised unless you have a real good relationship with the client.
Anyway, I would stick with a minimal fee (I say 1 hr is more realistic that 30 min since it takes time to switch between projects).
Here is an option I have used before and it has worked out very well. Simply have the client pay what he thinks is fair. I have used this with a client who asked for a quick update and I told him to log into my paypal donation page and simply pay the amount he thought was fair. To my surprise he paid the equivalent to over 2 hrs and I would have billed him a fraction of that.
From experience I find that most issues like this can be resolved with a phone call.
"You can just hang outside in the sun all day tossing a ball around...
Or you can sit at your computer and do something that matters."