The “stock” code isn’t really for day to day use, it’s just for year end adjustments. Anything you’re buying for re-sale goes on “general purchases”, the sales income is “general sales”.
For the cost of postage my QuickFile has two different codes, “carriage” is a cost of sales code which I use when suppliers charge me delivery charges to send their products to me, whereas “postage and carriage” is an overheads code which I use when I pay a courier to send my products to my customers (and there’s a third code in the sales section that I use on sales invoices when I charge a customer for delivery separately from the cost of the actual goods).
I’m not sure whether new QuickFile accounts get all these codes by default, you might have to create them from the chart of accounts page.