[Elsdebo]

Unfortunately I have no experience with any of those methods.

https://ftid.io/boxing

Does this work with Zalando?

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My DNA worth about 700€ failed.

I was emailing and calling them daily for 2 weeks, threatening them and trying to escalate the case - nothing worked.

What possibilities do I have from here on?

Any FTID version or LIT that might work?
Unfortunately I have no experience with any of those methods.

What did they tell you, why was your claim denied? You can always chargeback

They repeatedly told me the 'our shipping partner has confirmed successful shipping'.

I can't perform a chargeback because I ordered via invoice wich you have to pay manually.

I could pay it and then cahrgeback, but I only have one bank account.

Just don't pay. They'll delete your account maybe but whatever?
Their options are:
They keep sending boring emails to you, you deny delivery.
They might send you letter, but it has to be official one, then you can reply the same answer official and again deny delivery. Might even point them at certain consumer rights.
They might sell your 'debt' to a collections office. But the more I think about it, I don't think collection offices are interested in DNA's because it's even harder to prove for the collection office. Hehe suckers. These collection offices are a legal kind of social engineers tho, aimed against the small guy, and will bluff and lie to you, harass you with emails, calls and letters. And the papertrail it might leave on your name is not so good, it's sloppy SE.