Resident of Texas (not required to file state returns) - I have an Sole Proprietorship - I connect remotely to my client in Hawaii and perform work on their computers.

My question is, because I do file a Federal Return, in addition to an hourly rate I charge .08% in sales tax (as is on every receipt I get for businesses in Texas when I buy things).

1) am I ok to do this?
2) if so, what guidance or regulation can I send them to in order to back that up?

Thank you in advance.