You are able to create Line Items so that they are easy to select when creating or editing Invoices, by adding them in the Web Portal. Go to Settings cog > Accounts > Items > + New Item.
Alternatively you are able to add line items when you create or edit an Invoice on the Management Portal.
Note: this function is not available on the App. You will need to already have the items in your account when creating or editing an invoice from the app.