If you require additional assets that each package does not offer, you have the option to purchase extra files. You can purchase them during your normal logo check out, or at any time afterwards.
Purchasing add ons can only be done after checkout :)
Here is how:
Click on your purchased logo in your Saved logos
Scroll down to the bottom of your Brand page, to the section titled Design Add-ons
Select an add-on you would like, then click Purchase
On the Add-Ons page, select any additional add-ons you may want, then click Continue to checkout
Check out with your add-ons, you will only have to pay for the extras you added on, instead of your entire logo again