Website management cost?

I design website with only HTML/CSS without any PHP backend control panel, or any CMS etc, so when I need to update clients' websites, I need to do so by manually editing the html document. Since I cannot provide a CMS for my clients, I'd have to impose a fee for managing their websites. Any suggestions on what method I should use?

1. Whether I should charge based on time ($xx.00/hour, $xx.00/month).

2. Or, if I should charge them per event of website updating.

The problems I found so far:

If I charge the clients using any of the two options, they, very likely, will pile up a mountain of updates one time so it would be cheaper and easier on their wallets. (For example, if I use method #2, they might gather a bunch of tedious tasks, and give it to me at one time; This makes it difficult for me since it will consume a lot of my time for a small sum of money)

However, if I use method #1, clients may feel insecure about how much time I am actually spending to update or make changes to the website.

Any ideas?