My suggestion would be,

"Thank you so much for offering to take us to the airport, but I have already made other arrangements."

About the bills: Are they due soon? Can you do it after the kids go to bed?

If not, I would add, "I will get the bills scanned as soon as the kids are with you. Sorry that I can't do it sooner!"

JMO...

M 16 yrs, WH62, P54
3 adult blended kids
EA 11/13, BD1 6/14
PA fall 14, BD2 2/15
Piecing 2015, BD3 12/15
Separated 4/16
WH moved OW in 5/16
Divorced 6/15/17