If you click the “Add Item” button on a quote / reservation page, you will have the opportunity to search and add items. If they are available to add, they will show up as “Available” and you can add them to the Order and the billing summary will update accordingly.
To delete an Item off an order, simply click the trash can icon to remove the Item from the quote / reservation. If there is only 1 Item on the Order, you will be unable to delete that Item.
If the item has already been checked out or sold, then you won't be able to delete it, but you can set the price to 0 so that the customer does not get charged for it.