@karizmasalon Woah, 8 years?!  Thanks for sticking with us for that long!

Good question, too - You can totally subscribe to both!  I've seen other sellers so this to book their clients on the front end using  Appointments while tracking inventory and generating COGS reports on the back end using Retail. 

Note: If there are no unit costs associated with your items, then a COGS will not be available - as it's computed by subtracting the unit cost from the item’s sale value. Otherwise, you can still manage inventory on a simpler level without Retail - like track stock across locations and setting low stock and out of stock alerts. 

Here's a more in depth comparison on the inventory features offered with Square for Retail vs. the Standard POS.