Board Game Arena

We have the port with the bonus upgrade/3 VP which I can use. I thought that if I used the port I could upgrade a technology for 5 instead of 10 but the game is telling me it will cost 10. Which messes up my plans rather. Does it apply the update discount afterwards? Should I start the upgrade and then apply the port?
Any advise would be welcome.

Thank you.

You have to click on port then confirm that you are using it and then pick upgrade you want.