A lady with a long & deep review history here is not near as likely to cut & run as one from more insalubrious and less accountable venues.

 I'd err on the side of caution and give them the envelope after you have showered.

If its outcall - she comes to you - shower before she arrives.

If its incall, you're either visiting her house or apartment, or a hotel room reserved in her name.  Either way, she is unlikely to leave a stranger alone to trash the place after she's ripped him off.