Hello all. At times I put in a limit sell order and wait. My...

Hello all.
At times I put in a limit sell order and wait.
My online broker is an Australian bank.

For example:
sell 500,000 shares @ $0.005
It sits there for days and then one day boom, it executes.
sold 500,000 shares @ $0.005 - $20 brokerage = $2480 in my pocket
Life is good.

Unfortunately it sometimes goes like this:
sell 500,000 shares @ $0.005
It sits there for days and then one day boom, it executes.
sold 46,645 shares @ $0.005 - $20 brokerage = $213 in my pocket (453,355 shares remaining)
Life is bad. Equivalent 10% brokerage is breaking my balls.

Questions:
1) My broker has a minimum $500 trade limit.
But that only applies to me, not others buying from me, is that correct?
(If I try and buy or sell less than $500 I get an error message)

2) Do others on HC suffer from this same problem?

3) Is there any way I can stop this happening? For example:
- set a 'whole amount' trade limit
- change broker
- just not place limit orders

Thanks for your help!