I am told by the forwarding agent of container # GSU5445230 that my container has been pulled for random inspection by US custom, can the custom open the container in my absence?

Yes they charge for this. A lot of times they take it off site for inspection so you have to pay a trucker to bring it there plus the inspection and reload plus chassis and storage. If they have to bring it back to the port another fee for that. Nothing is free especially nowadays in 2021. Also nobody here on Google is going to tell you your container status.

If the custom inspect a container are there charges/fees to the shipper?

Yes they will inspect the cargo. And if past inspection.custom Will reseal.and released.check with your broker for container availability.