Whether you mark up the incidental items you purchase or not is immaterial

post the purchase to an expense account materials expense or something, and then what the customer pays you is income

on the P&L income is reduced by expenses

simple start and essentials do not have two sided items the way all desktop versions do, so if you use items for purchases (non inventory) then you will have to use a service item for the sale.  IMO it is easier to just post the purchase to an expense account, and then use the service item on the sale