Do you have your handling time set as 1 day. I think free post+ 1 day handling greys out that option (but I might be wrong).

Do you use C & S and if it sells say fri eve do you process payment, and hence get it marked as sent that day, even though its not going anywhere until monday? Buyers will compare date marked posted with date purchased. Also arrival time vs whats estimated by ebay, so getting that greyed out is best.

Do you street box lodge for sunday evening collection rather than wait for monday morning?

I always do a postal run for last collection in street box 6.30pm (nearest to me) rather than carry it over until next day.

If I cant get it in post with 24 hours i contact them and let them know, with apologies.