There are two ways that you can add Snapchat (or KIK) to sell on your account:
The first method would be to add it as a store item. To do this, you would click the drop down menu > My Content > create an item. From here you can enter details, a photo and a price as you wish. You will get an email when members purchase your store item, at which point you would PM them your Snapchat or KIK username.
The second way to sell your Snapchat would be as a service. From the drop down menu > My Content > Add Services > Texting
To add a service you need to have a minimum of 5 vids on your profile. This is the simpler method because once the member pays for it, he gets an email with your Snapchat name.
Hope this helps!