staying seperate from your spouse, married independence,

Today my husband and I hit some garage/yard sales in the area. One we stopped at had an older printer for sale for $5. Why would I want this? Only because my oldest daughter's laptop won't run a newer one, or at least that's what I have been led to believe. This, however, was the second $5 printer I have bought in an attempt to get her one. The last one was missing the power cord and turned out to be a wasted $5. I made sure this one had all the parts, but Dear Husband was NOT happy that I was walking down this road again.
Most of the time, I will defer to him if I think what he says has some merit, but if this has happened a lot he gets irritated by it. So, today, when he told me he thought it was a bad idea, something in me snapped. I bought it. Just a few days ago he was telling me to make a decision about something, and today he didn't want me to, or so I thought, so I made a decision I knew he'd disagree with. In discussing it later, while he was giving me a hard time about doing something stupid, (and yes, it was stupid. I can't get it to work) I told him to bug off because "you get upset when I won't make decisions, so you have no right to be upset because I made this one." I told him I felt a need to assert myself. I also pointed out that a $5 purchase was better than the time my mother did what amounts to the same thing. She bought a car. That was 30 years ago and I think my dad is still a bit bugged. I know he NEVER sent her to the dealer to buy a part for him again.
So, how do you keep your autonomy 15, 20, or 50 years into a marriage? You have got to make decisions together, and know when to defer them, and know when to make your own. How do you decide this? Personally, I make then when I feel they're important. Most of the time I just don't care where we eat, or what we do for fun. When I care a speak up, and if it works out, okay. If not then, next time. We take turns choosing movies, and we take turns choosing what to do when we go out, or where we go with the kids, and other things. There are things we just do my way, and there are things we do his. I put up with his family, he puts up with mine. I can decide to work late if his mother is visiting, and he can decide to play video games while my father is here. He does things he doesn't like for me, and I do things I don't like for him. Sometimes we choose not to.
At least I've never gone out and bought a car to prove it.