To stop paying extras for SC because their mum is being horrid?

I think a final email to set your stall out is a great idea. That way she can't plead ignorance. Well, she probably will do but you tried

Big no to whoever suggested the financial transparency with the ex. It's NONE of her business!

Can you and DH not respond “okay that’s fine we will take kids shopping on our weekend, then no need to send cash and receipts. Thanks.”

Kids school trip coming up “Thanks Suzie, no problem. DH will contact teacher to make arrangements for payment.”

I think saying this the satisfaction will be yours as you will see her face as the penny drops and she realises she has shot herself in the foot with her silly spiteful games.

I completely agree with you blocking her and refusing to cover more costs.

if it was me though, I would be sending her one last email saying you have tried your best to maintain a civil relationship with her, you have covered additional costs that weren’t yours to cover but due to her continuous alienation of you and your DH through her lies and bad mouthing, you will no longer be communication with her and are now blocking her. You’ll continue to pay for violin and football but anything other than that is between her and your DH. At least then you have a record in writing for the time when court comes around as well.

I’d think about actually taking the children to buy the uniforms, that way they see who is paying. Whether that’s you or their father (or both).

Stop paying the extras, pay her CMS level. And then when the SC are at yours, you can buy them clothes, etc. You can also then ask if they want to do x or y club and sign them up when they can see it is you making the payment.

(DH took this approach when it turned out his ex was claiming everything was from her)

What a horrible situation to be in.
In your shoes, I would start by handing over all communication with the ex to your DH. He can be the one who talks to her in future.
Continue to pay for the things you are - the music and other lessons.
I'd suggest that your DH reframes the cost splitting for items above those paid for by CMS as needing to be discussed first.
I'd be tempted in his position to point out that he and you don't bad mouth the kids about her because it would make them unhappy. She might feel resentful but it hurts the kids to be put in the middle.

@Withoutfearorfavour They are her kids, Hers and their Dad’s.

The OP doesn’t have PR for them and she isn’t financially responsible for them, is she?
Anything that she personally pays for is a gift to the children, not fulfilling a responsibility. Just as it would be from a grandparent, aunt, uncle, godparent, etc.

My DH and I are financially responsible for our children, no one else is. 🤷

Err this. She can get a job and pay for her own children rather than other people. My children are way younger than her and l have chronic health issues but still work pretty much full time

Yes - her kids and his kids but not the OP's kids.

What you are proposing seems fair. But have the conversation with the ex. (DH or you). Yes you are right. We are poor. We can only afford the CMS and the sports/music. Thanks for realising this and from now on you can do trips and uniforms and we will do the rest. Thanks for pointing this out to the children. They will be so relieved they won't suffer because you will be picking up those bills.

@AnotherNaCha to be fair she is subbing them. Zero chance of me doing that. I don't see why people don't pay for their own children. The ex and the husband to a degree are cheeky fuckers

Her kids. And there we have it.

No, don't punish the kids. Your partner is likely lapping this up, you two fighting it out whilst one financially carries him through life and the other looks after his DC whilst he waltzes in now and again to play Disney Dad and throw some of his wife's money at them.

Good. Then she can pay for them.

I wouldn't be paying for any of it. It's not your responsibility.

Tell the Mother to have one less holiday a year and pay for it herself.

You need to let her know that everything she says about you is being fed back and that since she has no respect for you, you have no reason to pay extras for her kids.

Then it’s high time she stood on her own two feet to fund the face-saving trips, rather than forwarding the invoice to you.

Not only would I be refusing to pay I would be blocking her email and number as well. I don’t speak to dh’s ex. It’s his job to deal with her. You’re just opening yourself up to a shit load of issues.

Just be careful. Latterly, I thought that I was getting on better with my late husband's ex. In our case, the kids were adults when she acquired her boyfriend, but she still managed to manipulate things.

(The final time that I had a conversation with the DIL, even she described the ex as 'jealous and controlling' - over the DIL's relationship with the son.)

The ex had me fooled. I actually thought that we were building a civilised relationship. I was wrong.

Or ‘f-off , stop emailing me and get a job’