There is a Chinese place nearby that DH lives. He especially loves their sesame chicken and eggs rolls. We ask for extra sesame sauce on the side for dipping the egg rolls. It has never been a problem and they are happy to do it. We would totally understand if they said no.
That being said, I used to work in a Chinese resturant. We would often get people asking for white rice instead of fried or to maybe leave the eggs or green onions out of a particular dish. Or even for extra onions or veggies. Always happy to do so. We did have one family that would come in and ask for these small changes and we always did it, and with no extra charge. They were a bit picky and got a reputation for being difficult customers. Then one day they ordered 2 orders of Mongolian Beef, hold the onions. Mongolian beef is onions and beef, period. So what they wanted was double the beef. I told them there would be a small charge for that request, since beef is considerably more expensive than onions. They were absolutely enraged. Substitutions should be free of charge, especially since the dish already had beef in it. They just wanted us to sub more beef instead of onions.
After about 10 minutes of abuse and them refusing to pay more for more beef, the owner agreed to make it without onions. Those were his exact words. The family smiled, very pleased with themselves. Until they got their order. Yes, they got the dish with no onions, but only the nirmal amount of beef. They started yelling again, but the owner booted them out, telling them they got what they had paid for and to please never come back.