Marianne doesn't agree to the request because she is really concern about the crown prince of England. She definitely want to do something more important. Of course she want to let the crown prince knows she is among his saviours, but she is more concerned about the trade in France.
There would be a time where sugar and spices will be scarce in England, that would be a year later. people from England have to go to France or Austria to get their sugar and spices. Most people that are not rich would only eat a tasteless meals.
Yes, she is following the crown prince of Austria to France to save the Crown prince of England and also, to purchase a big amount of sugar and spices. Not that she won't get the spices she need here, but, when she tastes their sugar for the first time in her past life, it is very different to the one here.
