Teaching Responsibility...????

Lately I've struggled with helping our children learn responsibility. Specifically, when they forget something they need at school and call me asking me to bring it to them. A dear friend of mine from Provo used to let her kids (high school age) walk to school if they missed the bus, or she would charge them $5 for a ride. I've thought of her often lately, admiring her strength as she must have given her kids a hug and sent them out the door to walk (quite a distance) to school..Rain or shine.
I considered how to teach Cairo responsibility and organization when she forgot the valentines for her classmates on February 14th and then on Friday, February 18th forgot some homework and both times called from school to request my delivery of such items.
I'm a stay at home mom. I struggle with what part of "Mom" means I deliver forgotten items to school. If I were a Mother who worked outside the home, it's likely I wouldn't be able to break away to drive home, collect the items and deliver them. As all stay at home moms know, I WORK...AT HOME...all day long.
In the case of Cairo's valentines I said, "I'll see if I have any errands down that way today and if I happen to be in that part of town I'll bring them." Seriously, we live in a town where everything is about a 5 minute drive from home. So, that sounded mean as I said it. I COULD make time to take the valentines to school but I wanted to teach her a lesson..Tough lesson, ON VALENTINE'S DAY.
I ended up taking the valentines and letting her know I happened to be in the area and it was lucky I could bring them..
When, later that week, she called again, I thought I truly must have failed on Monday because she made the same mistake right in the same week.
I gave her the same line about not knowing if I could bring them because I have a busy day and that wasn't in my schedule...
When I took the homework I also spoke to her about how, if a friend or neighbor called and was in a bind I would likely find a way to help them. But, I'm not her friend OR her neighbor. If, said neighbor did request a favor and I was able to help, certainly that person would feel gratitude and likely send a thank you in some way.
Today, Sam came into my room about 40 minutes prior to leaving for school and let me know he needed a shoe box for school today...
I don't keep such things around but Charlie recently got a new pair of shoes and was keeping some things in the box. I asked Sam to ask Charlie if he could have the box. Of course Charlie said yes because Charlie would give anyone anything, always.
About 10 minutes after Sam left for school he called and let me know he left the shoe box home..hmmmm. What to do??? I do go right past his school on my way to the gym but how will I teach him to be more conscious of his responsibilities?
I gave him the same story I gave Cairo about not knowing if my schedule would allow it and if I had errands that took me that way, I would try to find time to drop it off.
Sam is 12. I decided when I delivered the box that perhaps his lesson would have more impact if there was a slight embarrassment associated with it..Not a lot, but a little.
When I arrived at his class he came to the door to retrieve the box. As he walked away I said, "Wait, I need a really special thank you for going out of my way to help you today. How about a big kiss and a thanks in front of your class?" (I would not have done this if the teacher was in the middle of a lecture, but the class was working independently and had already been totally interrupted.) So, Sam gave me a kiss on the cheek and a big thank you to the cheers of his classmates.
We will see how that lesson works.
Any suggestions for future issues?