I'm trying to understand the use case here. When you say recurring payment for express checkout,it has to be something a customer is buying regularly like restocking an item, in that case it's not an express checkout, it's a recurring purchase by a returning customer. They must have opted to restock at a regular interval like every 30 days, 2 months etc. They also must be registered users on your website.
So your logic must include — Checking all the customers who have set up an automatic purchase for an item.
—Check their frequency of purchase and charge them for the amount of the item at that frequency.
—For this purpose PayPal has a REST API :
This is the accurate place to look for to implement PayPal recurring payments:
https://developer.paypal.com/docs/api/payments/#payment_create
In case they also allow you to authorize payments through the following API
https://developer.paypal.com/docs/api/payments/#authorization
you could use it as well.
But at least this narrows down your choices in terms of regular payment authorization.
This API will allow you to request PayPal services to authorize payment on a regular basis for that customer. You will still have to have your own logic to check the 'subscription' for your products and the intervals.
—The CreateRecurringPaymentsProfile API Operation (NVP) at the link below looks to be for personal payments. For example something an Inventory Management System will do.
https://developer.paypal.com/docs/classic/api/merchant/CreateRecurringPaymentsProfile_API_Operation_NVP/
It's an API specific to adding recurring payments functionality.
Since you're a developer, PayPal Developers' Resources will answer any queries you have regarding the API functionality and usage. You will have to just know what to ask for. You are their B2B customer trying to understand their API system. You can also file a bug on their Developer portal if you're not getting proper answers.