Dear Carrie,
This is a tough situation and one you need to get a grip on. You are right to be concerned about how this may carry through with your son later in life.
I always told my kids, from the time they were very, very little, it was always best just to tell the truth. All people make mistakes, accidents happen, things get broken...it's best just to tell the truth. They still may be in trouble, depending on the situation, but they would be in far less trouble for telling the truth than if I had to find out they lied to me on top of it.
The biggest and most serious consequence of not being honest is that people don't have to believe or trust you anymore. They can think once a liar, always a liar. Trust is the most important and valuable thing we can have with other people. Once trust is broken, it's very difficult, if not impossible, to ever earn it back. At 9 years old, your son is definitely old enough to understand that concept. You need to make sure he realizes that this is not about the $10. That's just money. The point is that he blatantly disobeyed you and then blatantly lied to try to cover it up. (Not even a good lie for heaven's sake. You KNEW there's no way he paid $9 for a postcard).
I would just really make sure he understands that he is going to have to earn back your trust. First of all, since you can't trust him with money, no more money. Certainly not for treats. Pick an amount of time and stick with it. If they have another field trip and you want him to be covered in case he really needed something, IN FRONT of your son, give the money to the daycare teacher/provider and explain the rules to them, since your son is having trouble following rules. (I would call them in advance and let them know so they can be on the same page with you about it, wink wink).
It sounds like your family is really growing. Congratulations on that! But your son needs to know that you will never be too busy to check and confirm that what he tells you is true, every single time! And there needs to be a punishment if you catch him being dishonest again. The only way for him to regain your trust and his privileges is to be able to demonstrate that he understands the importance of honesty. If he has a TV in his room, take it out. If he has video games, put them in the attic. If it was me, I would say he is allowed to do chores, (and make a darn good list of them), or he can read. Two weeks. That's it. Period. After dinner, he can help with the dishes, take care of pets if you have them, take out the garbage, take a bath then to his room to read until bedtime. He goes to daycare so it's not like he can't play and do things there, but at home, put his little hiney on restriction. And do NOT back down. If he gets mad or doesn't like it, talk to him, calmly, about why he is being punished and let him know that the punishment will be doubled if he lies again. You still love him, you don't need to yell and preach at him all day, but he needs to feel the sting of it and know it's all a direct result of his own actions and choices.
I have friends who have this problem with their son. They call my son to verify what did or didn't happen in class, on the playground, at so-and-so's house, etc, because they don't trust their own kid. They know my son won't lie to them. Their son continues to lie and they know he does, but they always cave in and he never has to "pay" for it. They are both well educated and professional people, but have trouble following through with discipline. I love them dearly, but they are not doing their kid any favors.
Again, make sure your son is reassured that you will NEVER have your hands too full to find out one way or the other if he is telling the truth or not. And you will check every time because you love him and want nothing more than to be able to trust him again.
Blessings to you and your family.