[mothered]

it's 3 t-shirts and they weigh about 150 grams each. I couldn't find an exact weight but it's something like that

The weight Is borderline In terms of the missing Item method.

It'll definitely work for those carriers who round up the weight to the nearest figure on consignment.

I've ordered £350 worth of stuff and said I only received a £20 t-shirt. Easiest company to SE

I said the t-shirt was missing and she said she would start an investigation and she'll keep me up to date on my e-mail, is this normal for them?

UPDATE: they want me to fill in some legal form about what items went missing, should I do this?

When using the missing Item/partial method, It's a commonality for companies to open an Investigation with the carrier and cross-check the weight of the consignment.

In terms of the legal document, Is It a statutory declaration or an affidavit? If It's the latter, It's admissible In court, so do not sign It.