QUESTION: If I do not need to track who has paid an invoice but just create a link to Authorize.net for let's say $50 which can be used to make a payment by anyone who clicks on the link, ie, I email the link directly or embed it into an HTML page.

In other words can I reuse the link generated by an invoice or is that link dedicated to that invoice only and if it's paid cannot be used to pay online again.

It would be a GREAT feature if Zoho Invoice would allow you to create "generic" payment links not needing an invoice.  Perhaps Zoho Invoice could have a module that generates not invoices but HTML landing pages with payment options to Authorize.net gateway.  
George
YourBizzWizz