Hello ! In your examples, "beaucoup" doesn't refer to the same kind of words, so that's why it's different.
In your first sentence, "beaucoup" is related to a verb. Plus it's "passé composé", so in this case, its place has to be between the "auxiliaire" and the "participe passé".
In the second sentence, "beaucoup de" is related to a noun, here "articles" so it has to be after the verb and before the noun it relates to.
Hope it helped!
Let me know if you have any other questions.
Some examples :
Ce midi, j'ai beaucoup mangé. (beaucoup is related to "avoir mangé")
Ce midi, j'ai mangé beaucoup de cookies. (beaucoup is related to "cookies")