Indy,

I'm familiar with setting up domains but as you imply the DNS settings can be quite difficult. If you (or other garden centres) just need to set A Records to point a domain's web traffic to the hosting (or to a Voice site) then that's easy enough.
If they need to set up MX Records to use a mail service like Office 365 online then that does require exact instructions for maybe 6 different DNS parameter settings.
Either way I would be glad to (try to) help! 
I do run a website design business www.minimwebsitedesign.co.uk but at my age am not taking on much new business and my fees would be modest, along the lines you suggested in the Zoom chat.

Best regards,

Roger.