You can. All of these options can be found by clicking on the Settings link at the top of the page. Then, hit theInvoice Details button in the sidebar on the right.
For late fees, select one of four options from the Late Fees pulldown menu: No late fee, 1% per month, 1.5% per month or 2% per month.
Check the Sales Tax box if you want sales tax applied by default on every invoice. (Late fee and sales tax options can also be changed on each invoice, regardless of the defaults.)
If you need to use a different sales tax rate, click on the Change this link and type in a new percentage.