Is custom plan available for both Zoho Books and Zoho Invoice?

Yes, custom Wave plan is applicable in both Zoho Books and Zoho Invoice. Once you login to Zoho Books using the referral link, the custom plan will be automatically updated in your Zoho Books organisation. However, if you wish to apply the custom Wave plan in Zoho Invoice, you will have to contact support-wave-uk@zohobooks.com to enable it in your organisation.