If you have a good relationship and can do it in a light-hearted way, sure. I still have my former names on a few things and they aren't easy to change.

I know it must be weird to see "your" last name on her account, but, legally, she has every right to keep that name, too. A woman is no longer required to change her name to her current husband's name.

As a matter of fact, I disliked my maiden name and the last names of my 2 former spouses enough to change my name again to a surname that I like. If I ever marry again (unlikely), I will not be changing my last name - never again.

And I hope my daughter never changes her last name, either. It's a PITA.

I would ignore it. It's probably just an oversight not a deliberate dig at you. I have a couple things in my maiden name because it's a pain to change everything under the sun and you forget about little things like this. All the paperwork that matters is in her new name it sounds like so this is just one of those things. Pointing it out seems like nitpicking and may come across as insecure. No need to sour things if you two get along. It might feel odd to see but it's a reality that she used to be with your SO so this is just a chance to show acceptance of the past and demonstrate confidence in your future. what matters is who you are now not who she used to be. Don't waste your time on it

I'll be Ringo, John and Paul and sing "let it be, let it be...let it be oh let it be"

BM still goes by my DH's last name.  It's the same name as her adult kids.. and I'm guessing she kept it when they were young due to it being cheaper to not deal with changing it.. laziness and appearance of having the same name as her kids.

I kept my Exh's name until I got remarried.. just a pain to change it pretty much.. and I didn't care really.

Nope, do an Elsa and let it go. 

BM divorced DH 10 years ago... yep TEN. She has been in two "serious" relationships since. Both live-in boyfriends and almost married. But she still keeps DHs last name. It used to puzzle me and I would stew over it, but at the end of the day, she is still the ex, so why should I care? 

I have placed an order on Walmart.com and went to go pick it up in store and it was under my ex husband's last name.  My ID is under my maiden name so I couldnt pick up my item.  This has happened to me at a couple of places.  I've had to go into the app and add "second authorized pick up person" and enter myself with my maiden name that matches my ID to retrieve my ordered items picking up in store.  So you might want to give her a heads up that this could happen but not from a place of "it's my last name now!" 

I'm of the opinion that you want to tell BM about it because it bothers you. It feels like you're sharing your DH with her and don't have him exclusively to yourself. You try to pretend that you're okay with this enmeshment when the reality is you are so NOT okay playing Happy Blended Family with BM and her current H. 

It's great to have a cordial relationship (some of us certainly don't!), but there are no boundaries like you'd have with couples who are merely your friends. You and your husband need quality couples time. Not constant combined couples time with his ex and her new H. It's very much okay for you and your H to spend time with the skids separately from BM and her H. And vice versa.

In response to your question... you would tell a friend about it. Since you have a cordial relationship with BM, tell her. As NoWire said, let her know pickup could be an issue if her ID doesn't match the name on the order. 

Nothing that I would be worried about. StepHell brings so much toxic stuff that this is a tiny blip that doesn't need addressing. 
 

If she has issues on her pick up, that's her issue to fix. 

If you have a good relationship with BM I would just let it be, if you have a typical friendship where you can cut up with them/joke light heartedly I'd give her the order and Say Here you are Mrs.(husbands last name) and laugh.. but I'd honestly just as others have said here let it go.. 

I had been divorced from my ex husband for about 6 years until I had all my information changed over probably just one of those things she either has not got around to yet or unable to change on her account or credit account.

Not okay. My XW was a bitch from hell in our final divorce hearing when she petititioned to return to her mainden name. Fine with me, I did not want that serially adulterous skank whore polluting my family name one second longer than necessary.

After she left, I started returning her mail that came to what was formerly out home with a written message of "NO SUCH PERSON"

When my bride and I bought a home about 13 years ago, we kept getting mail, packages, and collection notices for a prior resident.  We also got a ton of collection calls.  People could look up our phone number from the address information.

Eventually the former resident called to ask if a package had arrived for her and asked us to forward it to her new address. I took her number and her address then gave them to every collections caller, every lawyer, etc....

After that, her crap went in the trash unopened.