I feel I need to add one more module to the pre-existing Deco M4 3-module pack I purchased in December.  However, Amazon now seems to only have the "1-pack" [a single module] from third-party sellers only for at least $150.  

Ironically however, Amazon is still selling the same Deco M4 THREE (3)-module pack I purchased a month ago for $139....

Is there any reason why I can't purchase a new $139 set and simply use one (or maybe later even two) of the modules to extend my current system, or would this new 3-pack require some new, fresh internet service or address or whatever?  I merely want to add one of the modules to my current setup.

Thanks!