I need to send an ipa to the client to submit it to the App Store (he has given me his .p12 key and certificate, together with the provisioning profile).
When it was XCo
You must build for device using the appropriate certificate/provisionning, Then go to products, you find the .app file, choose "show in finder" Zip the .app file and send it to your client.
Once you've done this, your client must use "Application loader" to submit/updload the binary/application.
"Application Loader" is an utility installed with Xcode.
In general you find it in the path "xcode/Applications/Utilities"
Your client could use the application loader to upload the ipa.
You should first select Deliver Your App
and not Open Package
which is misleading.