Translation:They will have sent the package before you know it.
Dukaren and Hemiceni; since there is no Future Perfect in Danish, many different verbal structures (from other tenses or structures indicating other tenses; such ad kommer til at etc.) are borrowed to express this tense that we need to use since we have it in English.
English has the Future Perfect Tense: The stucture is will + have + V3, as you may know as well.
FP sentences in English fits to this form; but the Danish sentences borrow from Present Perfect and some structures such as "at kommer til at V1" etc. So, in order to translate an English sentence structured in Future Perfect, we have many options to choose from in Danish (all borrowed from other tenses).
As far as I have seen, one form or the other does not make a difference, as long as you follow the rules of the chosen tense or structure. So you can make all sentences in this lesson either with vil have V3, or with kommer til at V1, or others presented in this course. They all mean the same thing in English.
I hope this helps.