Forced to buy 1st class postage..

Hello, I have been selling on Amazon now for two years. I send small DL envelopes daily using 2nd class postage at 0.85p. I also work 12 hours shifts meaning that some orders for example come in at 7am, I am on shift so are unable to print and dispatch until I get home after 6pm. This has been fine, I have even had orders unfilled for 19 hours and am still able to send 2nd class postage. Up until this week all has been fine, however from Monday this week, any orders that are over 4 hours old, I am forced to pay for 1st class postage, from the 7th this went up to £1.65 which is eating into my profits. BUT even though I have an order that is over 4 hours old and am forced to pay for 1st class, an example is that if an order comes in at say 5pm ( I get home at 6pm) and I have missed the last post, I can still pay for 2nd class postage.. what has changed?

to break this down..

I can still pay for 2nd class postage at 5pm even though it wont be collected by royal mail the next day..

I am forced to pay for 1st class postage even though the 1st class order will not be collected by royal mail until the same time as the 2nd class postage order..

Crazy!