If you are using invoice templates from Re-Leased and have enabled the Pay Online button but it's not showing on the invoice, check the following:
Settings > Invoice Templates
Your custom invoice template with Pay Online enabled will need to be set as the default template to be used for all tenancies. This means the Pay Online button will be automatically added to all income invoices generated and created for all tenancies.
An individual tenancy
Your custom invoice template with Pay Online enabled will need to be set as the Default Invoice Template on the tenancy. This will mean the Pay Online button is automatically added to all income invoices generated and created for the tenancy.
An individual invoice
On each income invoice, there is a field to set the PDF Template. If your custom invoice template with Pay Online enabled isn't the default, change the template to be your custom invoice template and regenerate the PDF.