Can you create an issue for this?
http://code.google.com/p/openesignforms/issues/listWe can probably get it added to the next release as it does seem a small matter from a coding perspective. It looks like for a total sales tax we just need to include the param
TAXAMT when we submit to PayPal.
I do not think it has anything to do with tax liability, though. In general, you need to track sales tax yourself, make all appropriate payments to the state, etc. The PayPal tax amount is like the shipping amount in that it allows you to show a breakout on the PayPal invoice, but for most who are doing their own credit card processing, they don't even see such a value. Each state is quite different as it relates to sales tax, so calculating it can be tricky.
In Washington State, for example, we have different tax rates based on what is being sold, with many professional services non-taxed (a web application service is taxable, but hosting a web site is not -- so go figure!). We also have different tax rates based on addresses within the state, and we have to report and remit the taxes on a "location code" basis. PayPal has nothing to do with any of that.