Carly Opened Something That Got Her in Hot Water with Her Boyfriend

I crossed a line over the weekend. I opened something I shouldn't have, but curiosity got the best of me. It leaves me with a lot of questions about when is it appropriate to not have these kind of boundaries in a relationship? Is it ever appropriate? 

Some of my boyfriend's mail comes to my house. I have been really good to call him and say, "Hey this piece of mail came for you, do you want me to open it?" 95% of the time he says, "Just set it aside and I'll get it next time I see you."

A piece of mail showed up on Saturday that I was curious about. I suspected it was a bill and I suspected it was a bill that he was late paying.

I have trust issues when it comes to money and bills being paid. I'm a very responsible person when it comes to money, so I struggle when someone I am close to gets behind on finances.

I didn't want him to sweep the bill under the rug and felt responsible to tell him what the letter said instead of setting it aside, but I knew he wouldn't let me open it.

I ignored my conscience and opened the mail. He always says, "what yours is mine, I trust you with my life." I figured we were close enough that it would be okay, but my gut still knew it was wrong.

Later that day I confessed to him that I had opened the mail and my confession was met with very awkward silence. He proceeded to say "that's very uncharacteristic of you" and I felt like my parents were chastizing me. I knew I was wrong, but I didn't expect him to act that disappointed in me.

The incident has left me with so many questions? When is it okay to open your significant others mail? Is it ever okay? Should you have access to each others phones? Should you share a bank account if you get married? What should be kept private in a relationship?

What boundaries do you have in your relationship when it comes to finances and was I really in the wrong for opening his mail?