Shopify POS is pretty limited imo and not meant for a big business like the one your client has with the 39 total stores. I dont know what you guys sell or how big the inventory is but I use the POS system for my two locations and its the most basic pos you can have. You cant created bogo discounts for the pos, or block certain areas of the register such as total sales, no register open counter, etc... Also my one store has around 6000 variants and now the system acts up here and there because depending on which model ipad you use, their is a limit to the skus it can handle comfortably. On top of that you will get a bunch of random card reader disconnections, barcode scanner disconnections, receipt printer disconnections, card reader has completely malfunctioned and had to be sent back twice, things like that. I would never recommend this POS for a large operation like the one you mentioned...I think its really meant for smaller boutique type stores. In the end though I feel like shopify will continue to improve the pos and so that is why Im sticking with it for now.