New orders stating ship by today

I sell in the books category although some knitting patterns seem to be in home and kitchen. I use the Amazon template for postage which I have never changed. On new listings I always but shipping as 2 days but usually send out the following day. I always buy my postage through Amazon. Just recently some orders are saying to ship the day of ordering and not giving me the option of sending second class. I am being hit by late dispatch if I am unable to post the same day as the order is received. This is not all of my orders but is a significant amount. Has something changed on Amazon?