Concordance "de"

The use of "de" truly escapes me. In this exercise, why does the "de" after "profiter des jouets" becomes "des" but in "plateau de fruits de mer" it does not? I thought that when the structure requires a "de" (as in profiter de qch or beaucoup de), the "de" didn't change to du or des (like un plauteau de qch), whereas when it doesn't require it, you use it like "some" and it does change to agree with the noun (like here with commander qch becomes "commander des pizzas").