If no one is upset then the chocolate will just fall into the garbage can.

Also in the last stage in Libya, the chocolate will melt before it reaches the end of the conveyor belt, so only give it to someone who is within 3-4 positions left of the plate.

I actually played the game through before I even noticed the challenges, much less the conveyor belt. Guess I should start reading those little "notes" before each level...

I am on level 60 and having a heck of a time just passing it. I usually miss by 250 dollars and lose 2 customers. Ugh! LOL  I tried putting the candy on the conveyer belt in this level but it just instantly melts. Is this because you can't use this conveyer belt at this level or am I holding on to the candy too long or something? I just need a little push over the top to pass this level. any ideas? thanks! :)

The conveyer belt is only for samples.  When customers get inpatient or angry put any piece of chocolate on the conveyer belt.  It will go around the belt and then disappear at the angriest customer (you may even here a little click sound).  Once it disappears you will see the lines around the customers balloon change (really angry to angry for example).  It gives you a few extra minutes before they leave by serving them a sample.  If no customers are angry or there are no customers to be served, the samples will go all the way around the belt and then drop into the garbage pail.
 
The best thing to do is upgrade to the bigger display table.  As you make mistakes, put them on display.  When you are running behind put one or both of the mistakes on the conveyor as samples to ease up customer dissatisfaction and in the meantime fill the correct order.
 
Hope that helps.

The chocolate melts due to the heat/climate at this stage.  So, the conveyor belt is pretty much useless at levels 51-60 unless you are appeasing the very first customer to the left of the plate where the conveyor starts.

You have to put the chocolate on the round pad that is at the start of the conveyor belt for it to get to any customer.