You're correct. Simple example here. Transportation costs are added to other costs, and the same analysis follows from there.
Transaction costs are not really different from other costs, economists just like to highlight them because they're often forgotten.
I would like to learn more about economics but I don't know where to start. Can lesswrong suggest specific areas of economics that are particularly useful for understanding and optimising the world? Specific suggestions such as reading lists and resources would also be much appreciated.