You can integrate several websites using the same account!
Basically how this works is that a separate xPub (extended public key) is needed for each website. This way each website's funds go to a separate wallet and you can easily identify the website you received payment from. Please follow the below procedure...
Step 1: Get a new xPub. How to get multiple xPub's from the same wallet?
In many bitcoin wallet apps, you can create multiple wallets within the same app or account, each offering its own xpub. Airbitz , Copay and electrum are few wallet apps that can do this easily.
Step 2: Set callbacks for each of the websites
You can go to "Merchants > Store > Add a new Store" and paste in your new xPub and click "Save changes".
Add a new store for each unique xPub key.
You can also add a "Tag" (a unique name) for each of your stores to help you identify the store. 
Setting callbacks for different xpubs
Step 3: API Key
All the websites will use the same API Key. There is no need for a new API key. You can view your API Key on the Merchants Page.