Why shipping cost was not charged?

I created a shipping cost extension that returns a few options to the shipping checkout page.
Everything has worked well for a few weeks: the clients have always been able to pick one of the options and OpenCart has always been able to include the cost of the selected shipping option to the charged amount.

Yesterday something went wrong: the extension returned two shipping options, the client picked one of them, his credit card was charged, we received the order confirmation, but the cost of shipping was not included.
One thing in this confirmation email that I have never noticed in the past was the last line saying "AVS response: No match - No Address or ZIP Code match".

I am confused because:
- The address is correct. In fact my extension used it to ask UPS the cost of shipping and UPS accepted it.
- If OpenCart doesn't like the address, then why does it proceed with the checkout?
- This missing charge was for only $28, but sometimes our shipping cost go be thousands of dollars. How can I make sure that either they are applied or the order does not go through?
- How/where does OpenCart uses AVS?

OpenCart itself doesn't use AVS, it will be handled by your payment processor. "AVS response: No match - No Address or ZIP Code match" means that the customers address and zip code they entered, which your store then sends to the payment processor, did not match the address and zip code for the card used to pay. It does not mean the address was wrong, just the one entered didn't match the one for the card.

This is something your payment processor checks and may have risk management tools where you can set to accept or reject payments like this.

As for the missing shipping charge. If it appears on the order in OpenCart, then this points to the problem lying with the payment module. Which payment module are you using and how in the code does it send the totals to the payment processor?

If could be a bug causing the shipping total not to be sent added to the final total sent to the payment processor. Or it could be that the shipping cost is sent as one of the itemised totals and your payment processor is overriding it somehow.

If the shipping charge does not appear on the order in Opencart, then this points to the problem being with the shipping module or extension.

> If the shipping charge does not appear on the order in Opencart, then this
> points to the problem being with the shipping module or extension.

It could very well be the problem on the extension that I have developed. I looked at how other shipping cost extensions work and created my own. When I saw that the label and the number were displayed and charged correctly I was happy and went to production.

Is there a spec sheet that explains how to return a shipping cost to OpenCart?
Why does my data format usually works, but it fails when the AVS complains about the zip code (it just happened again)?

I don't know of any guide on how to return shipping costs. As you say It's just a case of looking to see how the other modules work.

Could it be the address information is wrong of getting corrupted or deleted during checkout? This would maybe explain why both your shipping and payments are having problems. Have you tried to recreate the problem by looking at the orders which had issues? Try to match the order as close as possible without placing an order (or use payment test account), including things such as the web browser used. You could even log some debug information to help work out what is happening.