If you sell things other than flights and training, you can create a price list for these under Admin > Billing > Shop.
Shop products are available when creating an invoice for a customer, see manual entries.
To add a product you will need to choose the applicable account and tax code. Therefore, if these have not been set up already, do that first!
Defining a product in this way reduces coding mistakes, ensuring the correct account code and tax code are recorded for each sale. When adding a product, you can choose whether the price can be changed when selling the product or whether it is fixed. This can be useful for prices that are constantly changing like fuel or when you have a special arrangement with certain customers for agreed prices.
When setting up shop products, note the description appears on the customer's invoice. The "item" is used to identify the product internally. If you have a lot of products available, it is a good idea to prefix the item with a code (if you use them) or a category such as "Book", "Gear" and so on. Doing this will save time when invoicing because you can type "book" to show a list of all the books you sell.
If you sell to the public, you might not want to record each customer's details. There is a "quick sale" option which you can turn on under Admin > Billing > Preferences. Once enabled, a new Quick sale tab appears in the primary links area. By default any sales are recorded against a special "CASH SALE" user account.