IMQ: Partner lying about getting a credit card

Hi everyone. I would like another perspective please. I'm a studying SAHM & have been for 2 years now. My partner is a dedicated dad, a loving partner & a good, supportive provider. We live on one relatively low income plus a small amount of weekly money from an inheritance of mine (until I go back to work part-time later next year). I budget carefully & we are left with $39 to spare at the end of the week for treats or whatever we've gone over in the weekly spend ;) Today a bulky piece of mail came for my partner from his bank. The envelope had come unsealed during postage & arrived open. I peaked inside (yes I know it was wrong but I did it anyway) to discover my partner has been approved to have a credit card with a limit of $2k. He hasn't talked about getting a CC with me. I sealed the envelope the way it should have come sealed & left it for him (looking back I don't know why I sealed it - it would have made this all much easier. I don't think I wanted him to know I looked as I felt guilty for looking & betraying mail etiquette). Later, I asked him it what was from the bank & he said that he had made some changes to his account to get cheaper fees & that it was information about the new status of his account. I told him I felt as though he's always very secretive about his finances in that he didn't talk to me about thinking of making the changes & he blew right up at me telling me it 'was all in my mind', etc & banged on about trust or lack of it on my behalf, that he hands over whatever amount I need each week, & that he never grills me about our accounts (& he never needs to because I leave the statements on the bench & the budget open on the computer for him to look at any time he likes) etc, etc. Apart from the fact I know what it really is, his complete unexpected over-reaction told me that something isn't right. I've caught him out before lying after a huge unwarranted over-reaction made me suspicious enough to do some more investigation. Anyway, before I met him he was shocking with money. No savings. Mega debt. Nothing to his name. (& I'm not convinced he didn't have a gambling problem but he does deny this). He came to live with me (I was financially well set up, house, furniture, car, no debt, etc) with his clothes, a bed & a washing machine. It took him 4 years to clear it all up (his money not mine) & then another 2 years (me working full time) for us to get into a comfortable position with money set aside (now our money) for bills, savings & holiday accrual for him so we can pay ourselves when he doesn't work, is sick or wants a week off (he's a casual employee) & to be in a position to be able to afford to live off one & a bit incomes for awhile. I don't know why he even thinks he needs a credit card? We eventually get everything he/I/we want even if it takes a bit longer. For starters I don't know where he's going to get the money to pay off a CC. We already have one that we use & it clears every month so we don't pay interest. I suspect he will only pay minimum off the interest for whatever he racks up. He's not getting any overtime lately for any extra money, nor is it looking likely. I can't think what he could possibly be wanting to buy that he'd need a credit card for over and above the one we have, unless it's for nefarious purposes (& before you suggest maybe it's for an engagement ring, he claims to have already bought me one & is just waiting for the right time to pop the question). Plus him having one will affect our credit ratings if we were to go for a loan if any sort (if he admitted to having it).... I'm really worried that he's going to get us back into financial debt again.... But, is his bank account (in his name only) any of my business over and above what he provides me each week with his pay? How do I say I know about the CC approval without giving it away that I've peaked at his mail? Do I say anything about the CC or do I just let it be, ignore it & hope for the best? Another perspective would be grand TIA.