Mac mini i7 trade in problems

I've just had a really strange trade in problem using Apple web site.

I've upgraded my 2018 i7 Mac mini to the M1 version.
I thought I'd use the trade in service from the Apple web site as it was really easy with my old iPhone.
I went through the process entering the model and serial number, ssd size etc and was offered £300
After receiving my new M1 Mac mini, the empty box turned up to send in my old Mac.
The box was much bigger than the Mini so I followed the instructions to pack it but also taped the mini down to stop it sliding around in the box. (I didn't put any tape on the actual Mac)

Today I received a message saying "Your trade-in value has changed" with the reason:
Product or serial number does not match your description

Now only offering £210 - I rejected the offer and contacted Apple.
They said the company, Alchemy, had said the serial number didn't match!
I checked the serial number on the photo they sent me of the back of my old Mac mini and it matched the one on the original box I still have and also the one on my Apple Care+ email. (There was no damage during delivery btw)

The Apple colleague didn't understand why the trade-in company said there was a problem even though the serial numbers clearly did match.

Apple have now asked them to re evaluate the trade-in value and will let me know within a couple of days.

My questions are; has anyone else had this happen to them and should I be worried that they are trying to scam me in some way?
I'm really surprised that a company trusted by Apple has done this.
I'll admit it has really spoiled my experience and I doubt I'll be using Apple trade-in next time.