You can add a parameter to the express checkout submission to prevent paypal from allowing users to update the shipping information from what you have passed in.
You can update the WA_eCart/PP_ECO_Scripts/PP_ECO_PHP.php file and update line 76 from:
"<LandingPage>Billing</LandingPage>";
to:
"<LandingPage>Billing</LandingPage>".
"<AddOverride>0</AddOverride>";
that should do the trick.