Customer checked out without paying shipping???

Website is http://meridianbotanicals.com. PHP 5.6. ZC 1.55 f. Alto theme.

Using USPS module to calculate shipping cost. I tried entering the same address details the customer used and was unable to advance to checkout without having the shipping cost be included with the total sale price. No luck. I have no idea how this person did this. I recall this happening on my previous site a single time, however I do not believe it was the same customer. My clientelle are mostly computer naive, so I am dubious that this is a hack or a crack, but rather just some odd coincidence.

Any ideas?

What payment methods do you offer on your store?

Does the customer happen to have two or more addresses associated with the account? Shipping option restrictions based on product selection or payment selection?

are you by chance using Numinix one page checkout? Some versions of that mod have a bug that resets the shipping, which can lead to the scenario you are describing.

We do processing with a merchant account. The payment shows up like this, totally skipping the shipping:

Sub-Total: $86.00
: $0.00
Total: $86.00

Shipping:

As if no shipping method was selected.

I do not see multiple addresses for this customer, but I am not sure how I would check exactly. The customer has only one address when I view their account from the backend. No shipping option or product restrictions.

No numinix or one page checkout either.

What is weird is that this happened once on my old site. I do not think it was the same customer. My old website was not used to make the new one, I basically rebuilt from scratch... which is doubly odd. New hosts and everything. The only data I migrated was customer account info.

I wonder if there is some odd weight number that would not give a shipping option... but I was not able to recreate the error with the same product weight. I moved the weight up and down a lot too and was always given a shipping option. It wouldn't let me checkout without selecting one either. hmmm

What shipping module(s) are available? Again, does a shipping option change based on any criteria or is everything pay one rate to ship all things regardless of what is ordered, where it goes, or how it is paid for?

See. My guess is that it is related to the following solution:
https://github.com/zencart/zencart/commit/d6db105053a98338342432cdb25ef842a2d5ea43

You know it may very well be. Out of 20,000 sales it has only happened twice, so I am not too concerned, as long as people do not try to game it.

I am using the USPS shipping module. I am just going to mark this one off as a mystery unless it resurfaces again. Thank you for the help.