Amazon Buy Shipping randomly not allowing 2nd class

Sold two copies of the same item in the night. For one of them, Amazon allows me to buy Royal mail first class post at £2.99 (fine) but for the other they’re only allowing first class (70p more).

This random thing has been happening for a while but it’s the first time I’ve been able to see such a direct comparison to this baffling anomaly. I charge postage based on second class so having to use first class is not good.

I only send in the UK so would could this possibly be about?