It's ages since I did any PayPal/eCommerce stuff but the way I remember it, the actions (at least from your perspective) are strictly sequential: Customer sumbits order to PayPal, including a contact email address (but not necessarily his PayPal email).
Customer logs into PayPal and pays for the goods.
PayPal sends confirmation to customer and forwards the order to the merchant (IIRC, there are options as to how this is achieved - HTTP, email or both?).
Merchant and/or his system acknowledges the order by email to the customer.
Merchant makes up and dispatches the order (progress may be available to the customer online depending on the sophistication of the merchants system).
This way, the merchant only ever gets to see orders that are fully paid for in advance and never needs to marry up a directly-received order with a PayPal payment. The merchant similarly never needs to be concerned with the customer's PayPal email address, which remains private to PayPal and customer (unless the customer uses one email addy for everything).
If I'm right then what you need to do is to:set up the merchant's account in PayPal's "Merchant Services" appropriately (this can be quite extensive and can involve the establishment of hand-shaking between PayPal and merchant's system, eg for price verification or quantity discounting).
arrange for Form B to populate the appropriate field in Form A, but not be submitted.
cause your Form A to submit to PayPal as a regular HTML/HTTP form submission (not AJAX). I think I'm right in saying that you can include, in the form's fields, a URL to which the browser will return when the payment is complete/aborted (ie. a "continue shopping" page), thus allowing the customer to do everything in a single browser window/tab.
Sorry this is a bit vague but as I say I'm dredging the memory banks from several years ago when I last did this. At that time, there was any amount of detail in Merchant Services but it lacked macroscopic overviews of it's various ordering/payment models like I have tried to give above.
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