PayPal Adaptive Payments with single item - paypal

Question

I have developed a small API that uses PayPal Adaptive Payments to create a chain payment.
Here is my current JSON body that I send to PayPal:
{
actionType: "PAY",
currencyCode: currency_code,
feesPayer: "SECONDARYONLY",
memo: product_name,
receiverList: {
receiver: [{
amount: payment_amount,
email: primary_email,
primary: true
}, {
amount: payment_amount-(0.03*payment_amount),
email: secondary_email,
primary: false
}]
},
returnUrl: "http://example.com/payment-success",
cancelUrl: "http://example.com/payment-cancel",
requestEnvelope: {
errorLanguage: "en_US"
},
trackingId: product_id
}
It works perfectly. The full amount is sent to the primary user, and then 97% of the full amount is sent to the secondary user.
Although, I would like to each of my items so that their quantity is always 1. My site is dynamic and allows users to create items all the time with their own price, name, etc. I would like to mark the item as purchased on my site after someone checks out with PayPal so that no one can buy it again, hence removing the purchase button.
I understand how to do that with Webhooks (provided by PayPal), although what if two or more users press the purchase button on my website and are checking out with PayPal at the same time?

Answer 1

You would need to put your own contorl over the concurrency in this case, as PayPal does not provide this uniqueness validation upon payment attempts in adaptive payments.
Here's how you may put the control in the front-end
Generate a unique invoiceId for each dynamic purchase button on the front-end
Collect the payment details (recievers, amount, invoiceID, etc) with your purchase button, and post the payloads to your API action page (when user submit the purchase button).
In your API action page, validate & save (into database) the invoiceId field to avoid duplicated puschase attempts (placing a status of ordered), and then invoke the PayPal pay API call to generate the PAY-KEY. Put the invoiceId in the payload:receiver object for tracking purpose in the PayPal callback and transaction reports:
receiverList: {
receiver: [{
amount: payment_amount,
email: primary_email,
invoiceId: uniqueId,
primary: true
}, {
amount: payment_amount-(0.03*payment_amount),
email: secondary_email,
primary: false
}]
},
Buyer is redirected with the PAY-KEY and completes the transaction
Webhook/IPN triggered and you'd mark your database entry as purchased
Note on Step#3,
In this flow there're actually 3 status of your purchase button: 1-available, 2-ordered, 3-purchased, this way you have control over it always as a single purchase
Incase buyer drops off(canceling or AFK or closing browser) in Step#4, you may clear the ordered status after 3 hours (PayPal PAY-KEY lifecycle)