After you have spend RM30 in a month, "Pay as you surf" is true.
Say Yes if you are using about 1gb a month.
Say Yes if you want to spend at least RM30 per month (to keep account active).
Say Yes if someone is paying for you.
Say Yes if you want to use the latest product.
Say No if you watch streaming video in the internet.
Say No if you play games in the internet.
Say No if you are a online gamer.
Say No if you are a downloader.
Say No if you are a webmaster or forum administrator.
Say No if you are a blogger.

Here are the scenarios:
a) Your example, you used RM9, RM21 will be deducted from your balance to make it RM30 (RM9+RM21).
b) You used RM35 for the month. Nothing is deducted, just your usage of RM35.
c) If you don't used for a month, RM30 will be deducted from your balance.

Note. If you don't have RM30 in your balance, when you log-in, you go directly to the top-up page.

Can any YES4G owner tell me if its modem has the feng-shui problem as p1? You need to place the modem at a specific location in the house?

Each month whether uou used or not, a minimum of RM30 from your balance will be reduced.
Here is the scenario for each month:
a) You don't used at all, RM30 will be reduced from your balance.
b) You used on RM13 and RM17 will be deduced to make up the total of RM30.
x) You used up RM33. Since you have exceeded the minimum monthly usage of RM30, it is pay as you use. No extra deduction.

On the 1st month you top-up with RM50 and then you don't used it for two months.
1st month - RM30 is deducted.
2nd month - RM20 is deducted and you owe Y$G of RM10.
3rd month, you have to top-up at RM40. (RM10 and RM30). Your balance should be RM30.