Does an agreement in a chat count as a valid contract?
In most jurisdictions (and for most transactions): yes. Usually the only thing that matters for a valid contract is that there is a mutual agreement – whether that is in writing, orally, via chat or via sign language does not matter. Of course, having things in writing makes it easier to prove in court if there is a problem, so it's still advisable.
What steps could I take if they don't send the money?
- remind them to pay
- if they still don't pay, you can sue them.
Some juridictions have accelerated court proceedings for simple cases like this (e.g. Gerichtliches Mahnverfahren in Germany), otherwise you will have to sue in a regular court that deals with contract disputes.
But I have no names and I am unsure what they can or have to do after
they received the package.
This is going to be the main problem. It's no good to enter into a contract if you do not know who the other party is :-). You definitely need to find out who exactly entered into an agreement with you.
- If the sale is to a private person, find out their name and address.
- If the sale is to a business (seems to be the case here), find the official name and legal type of the business, and make sure whoever you deal with is authorized to enter into contracts.
Otherwise the contract will be hard to enforce in court if things go wrong.