Brews, I will assume by your location and the fact that New Zealand is clearly marked on the passport that you are the person being "wronged". That being said I will attempt to be gentler than I feel like being.
Let's examine token, but we will substitute "dollar" for token. I would be quite silly to send in 20 "bills" of varying denominations totalling $44 dollars when $20 was requested.
Now, let's examine "the customer is always right". This can be either a means of going broke quickly, or a way to rip off your customer if he insists that his transmission needs to be replaced when one of his tires have gone flat. This is an "americanism" that was coined at Marshall Fields and exported to the UK by one Harry Gordon Selfridge (an ex-Marshall Field employee), and was originally expressed as "never argue with the customer in front of other customers", a far more sensible approach. I have been in sales and customer service related business for 30 years and have had many occasions when the customer was a complete idiot! Somehow people have began to think "The Customer Is Always Right" is in fact a right, like freedom of speech or something.
You have certainly sent in 20 items. I (and apparently Airfix) see it as 44 tokens. Some of the items are worth 2 tokens, some 4, etc. with the total value of the 20 items being 44.
If I was Airfix I would have sent you one kit and kept the 24 other tokens. Airfix chose not to do this, but rather sent back the tokens (against their own rules) along with a shining example of how bad the education system of the UK has become. The way the "relationship with the customer is being improved" is that they have sent back all the tokens, at their own expense, hoping that you resubmit two entries, each with the correct amount of tokens, and receive two free kits instead of only one.
Now let's examine "How to Resolve Conflict and Complaints With a Business". I feel the creation of a web page that is written to embarrass both a company and it's employee, while attempting to hide your own identity is in poor taste. I wonder how you would feel Airfix made a web page exposing how you didn't understand the value of each token? Have a nice day.