Whenever you add a new cost or line item to a Quote or Invoice, the item will automatically be stored for you to quickly re-use in future sales and jobs. You simply add the cost and the price, and the next time you need the same item, the SimplyPayMe App will suggest it for you as soon as you start typing like you can see below:
From here on, simply tap the suggestion and the fields will automatically be filled with name and cost. If you want to add several items to your inventory at once, you can set up a dummy job, where you create an Invoice with all the items you want to come up in the suggested costs when you create a quote or an invoice. This way they will all be saved for future jobs.