What may be the edge cases for testing a 'Payment link in SMS' feature?

Question

We are testing a new feature that is about to get released in a few days and most of the testing has been done by the team.

• The feature in essence is about payment link being sent in SMS to end customers (hence it is super critical!)

• When the User clicks the link he is taken to our page with only one option that says 'Pay Now' else the User can close the page (or the
  browser tab)

• Post clicking 'Pay Now', the User is taken to a third party page wherein he can pay via any of the methods like Credit Card, Debit Card, Netbanking or Wallets

What edge cases can we execute to ensure seamless transactions and smooth payments?

Answer 1

From the top of my head:

• User does not receive the SMS at all or with huge delay.
• For any reason SMS has not arrived yet, user click to receive the SMS again, 1st SMS arrives, user pays it, in few minutes second SMS
  arrives, user may click on it again. There should be a message
  telling the user it has been successfully paid.
• Same as previous case, but the user clicks on the link 2 times.

Answer 2

In order to understand the problems a little better, plus possibly address a few other issues I would need to start with:

• Who determined that main cases have been covered and that you only need to look at "edge" cases? What is their definition of coverage and what is their definition of edge cases. What tests have they written already and what do they check?

I'd want to break this down two ways. Both are fairly standard and apply to most situations I see.

What do you have/want for unit, integrated, end-to-end(UAT) and performance/security. What are your general requirements in each area?

What do you have/want/need for tests that cover happy, sad, optional and compliance paths and situations. What are your general requirements in each area for the business. There are many separate questions here and I don't want to reply an encyclopedic answer here.

Requirements can be light and fairly informal in an Agile settings, working software is the guide.

One thing i'd certainly look at for this is different devices but that is just one of many questions I'd be asking first.