There are three types of things you can add to an invoice:

Item – can be tracked, if you check-box the track stock it will not allow you to sell more than the stock you have. You can go to manage->items and either remove the track check-box or increase the stock.

Service – cannot be tracked (check-box is disabled) but can be sold and bought with quantity and price per item/service

Other  - when you want to add an item/service that or other things you sell which have not been pre-configured.

There are some behaviours in this table:

• when you choose “other” it will disable item qty and price  and skip to amount
• when you choose an item, the price and tax code will be filled
• when you choose qty it will calculate the amount
• when you specify item price it will calculate the amount