I had intended to eat a box lunch this noonoday, but the store that sells box lunch storees was closed, so ....

combine with the next sentence.

So.... I have to eat mutton soup instead.

The small portion costs 15 yuan and the large portion costs 20 yuan.

No matter the large one or a small one, they were both sharedwhich size, they both came with two small free bake buns.

I selected the small portion with two free bun.

You don't have to repeat two free buns because you have already established that both sizes came with buns.

I think tThe mutton soup iwas really good, but the bake bun iss were even better than the soup.

You've already established that you are comparing the buns with the soup, so no need to repeat.

Besides the two free two, I bought another four bake buns againones, I also bought four more.

Usually the mutton soup is cooked with sweet potato vermicelli, but this time, I changed it to tofu skin and Chinese cabbage this time.

So iIt tasted good this way.

Normally, you would not want to begin a sentence with "So". You could either make a compound sentence with the first one, or delete it.

In short, I had a nice lunch, no matter the mutton soup or baked bun, bothand both the soup and the buns were delicious.