In this case I am clearly their ( travel) agent and have built up trust with customers that I am getting them a good deal and since not everyday is equal sometimes I do takah to do get them a the best deal but some deals I have to add on my own commission to make money off it. Also not some deals I need to add on money to make a profit before I process the deal so I quote the customer a certain price before.
For example I see an airline ticket for $300 and but this ticket I do not think I am making commission off of so I tell the customer I am charging him $320; after I process the ticket I see the airline does pay me commission. I am still allowed to charge the customer the additional $20 from the original price which I quoted him from before?
Yes you are allowed to charge them the price that they agreed to pay you for the ticket. It is understood that you are not working for free.