I may be wrong, but I feel that making this website https-only could only add potential customers. Making matters worse, trying https://invoicingplugin.com gives you a warning and redirects you to https://woodenracks.co.uk.
Hey thanks for pointing that out. That other site is a side project for a friend and I used letsencrypt certbot to auto-configure it. Assumed it was fine but obviously not! Will fix.