We are starting to use Marketing Cloud with Sales Cloud (and Marketing Cloud Connect between them). When the sync runs, it puts every Contacts I need in my Synchronized DE. That's fine.
Because our marketers are used to use another CustomerId, I created "marketing" Data Extensions with "CustomerId" as a primary key, and Subscriber relationship. This Data Extension receives data with a SQL activity (from our Synchronized DE to this Marketing DE).
It seemed to be a good idea to me, especially because marketers would be able to search their customers (in Contact Builder) using our CustomerId and not the Salesforce Id. You know, when you want to check one specific Customer, or perform some tests on your own Contact name... Also, it allows our Mobile App to send our CustomerId to Marketing Cloud, instead of a SalesforceId (and our backoffice doesn't know this SF Id, we would have to send it to the backoffice).
So, when we send email, this CustomerId is filled in AllSubscribers AND in AllContacts (in Contact Builder). So, let's say we have 1 million Contacts synchronized from SalesCloud (and directly sent to AllContacts)... then we will have another 1 million coming from AllSubcribers. And that's a matter of $$$ because our contract says 1 million Contacts max...
Do you see any workaround to this ?
Thanks a lot for your help !
Edit: It seems I really have to use the Salesforce Id as a SubscriberKey, or my Individual Email Results will never be filled in Sales Cloud.
Even if it make sense, it's quite ennoying. Here is a consequence I can imagine : If someone just subscribes to our website : we want to send him a transactional message to say hello. For now, we used to send our CustomerId in the payload, so Marketing Cloud will be later able to know who received this email. In the future, I would have to know the Salesforce Id linked to this contact. I would mean that our website has to create the Contact in Salesforce first, then receive the SalesforceId, THEN send the transactionnal message.
I would be very happy to have any confirmation from your part. I'm only doing deductions from what I'm now understanding.