The great thing about Zoho is that you can quote customers via a "share estimate link" (a kind of unique public portal) where customers can immediately accept your estimate. And if it is accepted, and automated, your customer will immediately receive an email with an invoice for x% of the quotation.

At this time, customers must walk away from the estimate to dive into their emails, search for the retainer invoice, think one last time over it and then make the effort to pay it.

As a golden rule to never let customers walk away from what they want to buy, it would be much more efficient to not only accept online estimates, but also to pay for it instandly.