1st of all, slapping is wrong.
A lot of problem with 'Chap Fan' or mixed rice is that the portion sometimes doesn't correlate to the price.

If the portion is standard like a piece of drumstick, no one will complain when the price is obviously displayed.

However, if it is small pieces, a more obvious example is to put a count on it, like '1 piece = RM1.50', so if you take 3 pieces of chicken from a curry chicken dish, it goes to RM4.50.

But it is also not realistic because not all pieces are the same especially for meat that comes with bones. Or maybe the cuts are not standard size. In this case the seller must either take the lost or weigh the dish (also not realistic for gravy heavy dish).

Same with vegetable, sometimes you take 1 scoop, it is RM2.50, some times the same scoop they will charge less, depending on how they look at it.
The BEST way to sell mixed rice, is actually how they do it in dim sum restaurants. That is the vendor will place all dishes into a small bowl that is standard size. Same like dim sum.

Customers will just pick up the bowl where the price is listed.

The downside is vendor needs to clean a lot of bowls. However workloads can be reduced with automated dishwashers which is common in restaurants.

Also need more manpower to scoop into portion sized bowls (at least 2 more staff) and more space required to place the bowls, which can be solved by stacking trays (by staff again).

Another way is to make it NON-SELF SERVICE. Means that the staff will scoop for you, which is also common.