I live in a rural area and DS goes to a school outside catchment. It means that DP and I are responsible for getting DS to and from the bus stop each morning and evening.
Our neighbour has a DS who has started at the same school this year and they approached us about lift sharing, as it seemed to make sense.
My DS is in year 10 and attends a club one evening per week where I pick him up on my way home from work, so I thought it would be fair for me to offer to do the morning lift sharing as there would be one night per week, that I wouldn't be able to pick up. I made this clear from the beginning.
With hindsight I was too quick to volunteer the mornings as it seems the neighbour is only able to do a maximum pick up of 2 or 3 nights per week. So we are doing 5 mornings and getting 2 or 3 evenings.
The other issue, is that the neighbour has only been telling us in the morning whether she can pick up or not that evening, leaving DP and I having to frantically arrange alternative pick ups or alter our work around.
So, I specifically asked neighbour by text if she will be doing pick up on a certain day this week, as DP and I can't do it without arranging an alternative, she replied that yes, she can, but she can't do 2 other days at all from now on.
I therefore suggested that she do 1 morning per week, as that seems fair to me that we are both doing 4 runs each.
She has just text me back and said that mornings are not good for her. 
So am I being unreasonable to go back and say that I only think it's fair that she does her share? Lift sharing is supposed to be just that, and that 2/3 lifts per week are not helpful (especially if I have no idea when they will be) and we are gaining very little as we are still having to be on standby in case she lets us down, whilst giving her 5 mornings a week to still be in a dressing gown whilst she waves her DS off.