recurring billing

I'm trying to implement recurring billing for my client on their fully functional zen-cart website.
I have searched everywhere adn everything and come up with the conclusion that it can't be done . I had no idea that it was that difficult as paypal does all the processing adn recurring and automating.

What if zen-cart was utilised normally as it always is until the point of 'confirm order' where the sale goes to paypal for processing. Instead of confirm order, it would be subscribe to order and rather than go to paypals 1 time purchase it goes to paypals subscribe page.
Does that make sense?

Paypal doesn't need to know what was ordered as zencart keeps track of that, paypal just needs to create the users subscription profile and process payment.
(And if I'm lucky return to clients website with success - hope IPN's are same)

Is it just a matter of changing the address that 'confirm order' button goes to, or is it so much more than that?

thanks.

I'm just adding some thoughts out loud, hoping someone will chime in with encouragement or tell me its a stupid idea because 6 other pages that I didnt even consider also have to be changed.

On the paypal process form which links to the confirm order button I'll:

Change the cmd name to value "_s-xclick"

Add to the form hidden items:
The subscription amount is the total amount (IPN value "a3")
The subscription frequency of billing per term is every 2 months (IPV value "2")
The subscription billing term is month (IPN value "M")

Change the IPN main handler to have these records also.

Can you see any problems with this method?

Oh I'll also ensure wording on the site reflects the change to subscription, change the button names to subscribe, include the updated info in t&c etc

Thanks

Don't give up. Here some encouragement.

If you are just looking to have paypal subscriptions, and the client only needs to know the original order purchase, and can rely on paypal to handle all future billing/orders. Then this would not be a huge project. Be sure to think about if the clients product is virtual, or if it is shipped, then you would require shipping and address information needing to be updated, etc. there may be confusion between where a customer should update this information, and information done in paypal will not transfer to your zencart customer details, and vise versa.

Now if your client requires you to retain subscription information on site, or if the subscription allows the customer to change products that he ordered, or upgrade his subscription. A few other reasons pop to mind. Then this can be quite a bit more difficult. I first tackled a subscription system back in 04, unfortunately its proprietary to that company and is based on CC and Direct Deposit payment systems.