How to verify non-renewing subscription

I am confused how to properly deal with the non-renewing subscription validation backend. My validation flow roughly like this: First, when user choose IAP item and click the

Buy

button, it will send a

POST

request to our backend for generating a order which will contain info like

product_id

. And then return the

order_id

to the ios. Secondly, the ios end call our receipt verify API which should be passed through some args including

receipt_data

,

order_id

,

product_id

,

transaction_id

. Thirdly, when above args passed, we query db with

transaction_id

for avoid delivering dulplicate goods with same receipt. Then, sending a POST request to the App Store endpoint

/verifyReceipt

. We parse the resp considering different style of receipt, get the

status

field first to judge if its value equels to

0

, then travesing the

in_app

field to find if there is one which

transaction_id

matches the one passed from ios. Then, we compare the

product_id

with the matched one. Is there something else that i should pay attention to or there are some logic bugs?