How do you know this woman advocates against you? Is this what your husband has told you? If your husband has had affairs in the past, then it's highly likely that he's involved with this woman sexually.
She knows the address of the flat that your husband stays in, yet she chose to send a card to his home address. If she'd wanted to keep their liaisons secret, wouldn't it have made more sense to send the card to the flat?? In my mind, she deliberately sent the card to the family home, and deliberately wrote the message inside, so you'd know she'd visited him at the flat (probably regularly).
Why does she want you know....well it's not out the kindness of her heart is it!! She's the OW, who's probably been having a relationship with your husband, and is desperate for him to leave you! So she thought, she'd give you a prod, in the hope you throw him out, and therefore pushes him to do what she wants.
Yes, you need to confront your husband. However, at the moment he isn't aware that you've found the card. You need to have a very long think about what you want going forward. Your husband is a cheat, it's very clear he isn't going to change his ways. Do you want to live the rest of your life with a man, who doesn't love you and has zero respect for you?? Do you want to spend it, constantly wondering if he's lying to you, what he's doing, where he is?? It will eat you up inside.
Personally, I'd be gathering as many financial documents, I could find and seeking legal advice. Once armed with that advice or possibly divorce papers, I would confront him.
You deserve much better than your husband, he's an arse.