But when you get to be my age things in your remember box sometimes don't stay for long , so I am asking again LOL

how long to do you have to mark books mailed after label is printed??

It all depends on what mail by date you chose.  You can choose the longest possible mail by date, print the label right away and still have 7 days to mark it mailed.  You can even reprint the label after it's been marked mailed.

My car is in the shop until one day next week, they can't say when because of the part they need to find.  If I wait until the last day to accept then I will still have a few days to get them to the post office. 

Yes Bertie, there's a two day window after you mark the books mailed.  BUT, if the car repair runs a bit long, you can pm the receiver and notify them of a late mailing.  Most members are understanding if you keep them informed.

After a certain time you would have to ask permission to send, that date is a week or two IIRC.