Do I Keep My Retainer Fee And Ad Spend Separate?

Yes, best practice is to ALWAYS make sure that the fee being paid to you by a client and the money being spent on ads are completely separate.

This is fairly simple to do with the billing systems we use. Clients pay you directly via invoice on Stripe, and they pay for their ad spend by connecting their card or bank account to their Facebook Ads Manager.

This way, there is never a conflict of interest of "withholding" ad spend so that you make more money from your clients. You get paid a fair, flat fee every month for the service you provide and they pay for their ad spend, which never touches your hands. 

This is how we run our business to this day and how some of the biggest agencies in the world run their billing, and is what we recommend all of our students model as well.