I strongly suggest that if you want to fulfill the mitzvos of tefillin and mezuzah properly that you follow a few prudent steps:
Purchase your mezuzos and tefillin from someone that you or your Rav know well. Tell the seller that you want high quality without any compromise – and that you are not looking for something “cheap”. If you cannot afford top quality for all of your rooms, you can save some money by asking your Rav which rooms are “questionable” as to their need of a mezuzah. You might be able to save some money by buying a lower grade of l’chatchila mezuzos for those rooms. Please pay as much as you can afford for top notch tefillin. Tefillin are forever. Make sure to take your son with you when you buy his tefillin. If he sees that there is nothing as important to you as his tefillin his Yiddishkeit and that of your future grandchildren will be given a major push in the right direction.