PostGIS in a commercial project

Hello,

we are using PostgreSQL in our projects and would like to integrate PostGIS as well.

Now PostGIS is licensed under the GPL and I wonder if we can use it in a commercial (customer specific) project then.
The source code will not be made open source, but of course the customer will get the source code.

Is it still OK to use the GPL licensed PostGIS in this case?
Is that then considered a derivative work because the application will not work without PostGIS?

Regards
Thomas