Rather than having to fill in often repeated details in a sales invoice's description box, you can use a dropdown list of items from the Stock/Product lists feature. As this feature is optional, make sure it's switched on in Settings > Configure system > Toggle features. Once switched on the feature is available via Tools > Stock.
Navigate to Sales > Invoices and select Create Invoice.
Alternatively, you can select the stock/product item on the Tools > Stock screen and by clicking on the Create Invoice From Selected button.
Select the Stock or Product item from the Choose an item drop down field above the Details field:
The detail of the item set up in the Stock tool will now populate the details field:
Note: in addition to the description of the item, you can also set up the stock/product item to populate the price, account, VAT rate and standard quantity on the invoice.