How to have a service dependant on an item in stock

[RepairPad]

OK. . Had square for a while only used on occasion for card payments now I find appointments and things are looking up.. but take the following scenario 

Customer needs to buy and item and book an appointment for that item to be fitted .. if the item isn't in stock can still book but will be notified appointment correction due to awaiting stock... but if item is in stock customer books appointment. Item is allocated and all is good..

So how without making the customer do two separate things.. or internally have to switch between apps or use pos, can this be achieved ?

Seems silly to not already have the link there