Currently we sell products for say $3000. The customer will pay a $1000 deposit/initial payment then be required to make regular monthly payments of say $400 for the next 5 months to pay the remaining balance. We have used 2 methods to do this.
A) This is mostly automated however the customer (and us) has to work out the balance owing manually (ie the balance remaining is not indicated on any invoice etc). What we do here is create an invoice for the initial deposit/instalment then create a separate recurring invoice for each of the monthly instalments that is emailed to the customer automatically a week before the due date.
B) This method is a lot more labour intensive and opens up a greater possibility of making errors as invoices are manually created each month. What we do in this method is create an invoice for the full amount with a due date on the last day of the final monthly instalment. We then create a 'Retainer' invoice (we customised the title to say 'Instalment Invoice') for each of the instalments (initial $1000 then each of the monthly $400 payments). We create and send these invoices a week before the payment is due from the customer. With this method the customer (and us) can see the progress they have made towards making the full payment for their product and is a lot more useful in the account reporting.

What we'd love to see is:
1) a way to create 'Recurring' Retainer Invoices - so we can use the retainer invoicing system for instalment payments
2) a link / button to click during the creation of a normal invoice to enable the creation of the 'Instalment Invoices'
3) a due date for the 'Retainer Invoice' that can be changed during the creation of the Retainer Invoice (like what is in the standard invoice)
4) the option to send a customer statement when sending the Retainer Invoice - so the customer can see how much of the initial purchase price they have left to pay and what instalments they have made so far
5) the ability to associate a Recurring Retainer Invoice to a standard invoice so that payments made on the Retainer Invoice are automatically assigned to the initial purchase invoice - this one is less important but could be handy if multiple invoices are sent to the same customer for different products/services that are not related to the invoice with the instalment plan associated to it.

There is a bunch of requests here so I hope a least some of it can be easily implemented.

Thanks for your work. Zoho has been a great product for us.