Basically, no. You could put your app on the main App Store, publish it over TestFlight or have your client get going with an MDM.
There’s no easy way to side load apps aside from MDM so unless they jump through all the hoops to enterprise sign an app and you hand them the build, they need to get going selecting an MDM to push the VPP codes from the B2B App Store.
Get them started with Jamf Now - it’s cloud hosted, has amazing support from the vendor and community and is free for the first 3 manaded devices so you can do a proof of concept before paying: