Here are the completed sentences with the correct words:

1. For online shopping, you just visit a seller's website, select the product you want, and order it.
2. Online sellers often ask you to pay for the cost of shipping.
3. One attraction of an open-air market is that it is different from place to place.
4. During holidays, a lot of people go to shopping centres to see the decorations.
5. Almost every big city has a department store with a wide range of decorated products.

And here are the answers with the correct adverb of frequency underlined:

However, the question doesn't seem to provide options for adverbs of frequency. If you could provide more context or the options, I'd be happy to help.

Based on general knowledge, here are some adverbs of frequency that could be used to describe each activity:

1. For online shopping, you **often** visit a seller's website, select the product you want, and order it.
2. Online sellers **usually** ask you to pay for the cost of shipping.
3. One attraction of an open-air market is that it is **always** different from place to place.
4. During holidays, a lot of people **frequently** go to shopping centres to see the decorations.
5. Almost every big city **normally** has a department store with a wide range of decorated products.