Hello @Massi,

Welcome back to the Klaviyo Community!

If you haven’t had a chance to review, I would suggest taking a look at some of our Help Center articles going over what and how to setup a dedicated sending domain to get a basic understanding:

• Understanding dedicated vs. shared sending domains
• Understanding what a dedicated sending subdomain is
• How to set up a dedicated sending domain
• Troubleshooting dedicated sending domain issues

But to answer your questions:

1. In terms of working with your clients and who actually does the setting up on the backend, that would be something determined between yourself and your client. If your clients are comfortable allowing you access to their DNS and you’re comfortable making these changes, I don’t see why you can’t make an argument to set it up on their behalf. Else, you’ll want to instruct your client to make those configurations within their DNS on themselves. 
2. If you’ve verified your sending domain with something like @store123.com, then you’ll want to stick to that as your sending domain. Any deviation such as using @send.store123.com would cause a misalignment since it’s technically not the domain you’ve verified. 
3. Once again, this would go back to how you’re managing your clients. I don’t see why you would need to be the one to be paying for their subscription - unless you’re invoicing them for the monthly recurring charges. But typically each client will be responsible for the monthly subscription plan on each of their own accounts where you’ll simply be added as a user to perform work on the accounts.

David