Hello there, @oliver2.

Thanks for joining the Community family. I'm here to provide some insights about the letter "T" that's showing up upon printing invoice in QuickBooks Online. 

The letter "T" that's showing on your invoice upon printing indicates that the amount is taxable. If the item used for the invoice is not taxable, then the letter "T" will not show up after the amount.

For more details about sales tax and sales forms in QuickBooks Online, please check out this article: Apply sales tax rates to transactions and sales forms. 

This amount of information should get you moving today.

Please know that I'm just a reply away if there's anything else you need about this concern. Wishing you and your business continued success.